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Backus–Naur form. A formal description of syntax used within a standard. The BNF description will show, for example, what characters can and cannot be used as a specific value. See more:


RESO Certification will test programs and platforms for conformance to RESO Standards.


A sub-category of a Data Dictionary Wiki Resource. For example, the Property Resource may have RESI, COMI, LOTL, etc. as Classes.


A software application which runs on a computer and relies on a server to perform some operations. For example: Microsoft Outlook may be your email client while Microsoft Exchange is the server which routes and stores your emails


A RETS client (RETS interfaces will be deprecated with Web API taking its place) can request that the results to a query be in COMPACT format. The most basic explanation is that this is a tab-delimited file and the idea is that the bandwidth requirements are less than for representing the same data in STANDARD-XML format. Lookup data is provided in it’s coded format. COMPACT and COMPACT-DECODED formats will generally require that the client software utilize the server’s metadata to fully understand the fields presented.

Compact Decoded

Used with a RETS Client (RETS interfaces will be deprecated with Web API taking its place) and similar to COMPACT format, COMPACT-DECODED results are usually tab delimited. The major difference between COMPACT-DECODED and COMPACT is that COMPACT-DECODED results will have Lookup data in it’s most human readable format–coded fields will be expanded to something that is generally understandable to humans. Like COMPACT format, the client software will need to utilize the server’s metadata to fully understand the fields presented.

Compliance / Compliancy

Once a RESO compliant client or server has been developed, a Compliance test can be run to make sure that it follows the RESO standards. The tests are defined and designed by the Compliance Testing Workgroup.

Data Dictionary

The RESO Data Dictionary serves as a national standard guideline for all information pertaining to MLS, Brokers and their Technology Partners. The Data Dictionary workgroup’s purpose is to keep current and maintain the Data Dictionary such that it is always relevant for those parties in need of such a tool.

Data Elements

Pieces of information delivered from a server after a query and may include field values, images, URL’s, etc.

Data Recipient

Person or entity which receives, pulls or accesses MLS content.

Database Name

The unique identifier the MLS database uses to name a field. Some MLS databases use numeric database names

Database Server

The “backend” MLS system server which houses property, agent, office information as well as images, documents, etc. A database server is proprietary to the MLS vendor and works in conjunction with the MLS system software


When an object is being phased out of its current use


Document Type Definition. These files are used to describe an XML document so a computer program can automatically “learn” how the XML is structured and what data types are used.


File Transfer Protocol. FTP is a file transfer standard for moving static files from one computer to another.


Applying rules to a User Agent, User Class, etc. which limits the data elements, amount of data, number of queries returned by a request for a specific User, User Class, User Agent, etc.


Graphical User Interface


A RETS request type (RETS interfaces will be deprecated with Web API taking its place). The GetObject request is issued from a RETS client to a RETS server in order to obtain objects for a specific listing such as property photos or PDFs. The request can be for a single, specific object or can be issued to retrieve all objects.


Hypertext Transfer Protocol. HTTP is the protocol the RETS 1.x family of versions use for communicating back and forth. Authentication is typically accomplished using either Basic or Digest authentication within HTTP. Request-specific values are passed to the server within the URL while the body of the response is returned back to the client in the HTTP body.


Internet Data eXchange. At it’s core, IDX is the concept of displaying property information from an MLS on an Internet website, although over the years, IDX has taken on different meanings. An “IDX feed” or “IDX file” is typically an exported file (once every 24 hours or so) that resides on an FTP server controlled by the MLS. “IDX vendor” is a term used to describe a 3rd party company that provides a product or service that makes use of data from an MLS.


A field that contains preset codes as it’s value. For example: PropertyType might be a field that contains RES as it’s value. Doing a ‘METADATA-LOOKUP_TYPE’ request through RETS would tell you that PropertyType can contain RES (the code) for Residential (it’s value), CON for Condo, LAN for Land, COM for Commercial, etc.


Metadata can be thought of as “data describing the data”. When used with the Web API or RETS interface, retrieving the Metadata for a specific property class (example: Residential properties) will return very specific information about all of the fields, field types, which fields are searchable, etc. A “Metadata-aware” Web API/RETS client can use this information to automatically recognize changes to a data format (like when a field has been added) and adapt accordingly without user intervention (by adding a new field in their local database to make room).


The MLS is a tool to help listing brokers find cooperative brokers working with buyers to help sell their clients’ homes. Without the collaborative incentive of the existing MLS, brokers would create their own separate systems of cooperation, fragmenting rather than consolidating property information.


Open Data Protocol (OData) is an open protocol which allows the creation and consumption of queryable and interoperable RESTful APIs in a simple and standard way.


Plugfest is an open workshop for technology partners to test their interoperability held during the trimester conferences. This is a popular event where some of the best technical staff from the industry meet to help each other connect using the RETS specification.


A query is made up of different sub queries separated for logical operators. Each sub query can contain Sub Queries, Lookups, Ranges and Strings.


Rule Change Proposal. An RCP is a document which includes a possible rule change to the specified technical document. Once voted on and approved, the change described would be implemented into the standard.


Real Estate Standards Organization, a body that is a legal United States Corporation whose purpose is to govern the standards development, promotion, and maintenance activities of RETS and related activities.

RETS (legacy)

Real Estate Transaction Standard. This term is now considered antiquated and refers to the soon be be deprecated RETS Client and Server.


An action performed against the server, such as a query, call for images, open house URL’s, etc.


The Data Dictionary term for a general category of data available through a RETS for Web API server. For example: Property, Agent, Tour, OpenHouse, etc. are considered Resources.


Simple Object Access Protocol. Provides a way to communicate between applications running on different operating systems, with different technologies and programming languages.


The structure of a database system, described in a formal language supported by the database. In a relational database, the schema defines the tables, the fields in each table, and the relationships between fields and tables. RETS Schema defines fields, data types and classifications.


The Server is the central computer system which contains the desired information. A Server handles requests from Clients, applies business rules and other limitations on those requests (through filtering) and provides a valid data response back to the Client.

The pipe “|”

Means OR so everything separated by commas has is OR’ed.

The plus “+”

Means AND so everything separated by a comma is AND’ed.

The tilde “~”

Means NOT so everything separated by a comma is NOT’ed.

Universal Property ID

The Universal Property ID (UPI) is a portable mechanism for uniquely identifying properties. RESO has formulated a basic concept that a Property ID be derived from the authoritative public descriptions. The UPI helps avoiding duplication of referenced properties when they are represented across organizations/entities/etc.

User Agent

A User Agent is the name a RETS client goes by and what version is currently being used. For example, a User Agent of RETSclient/1.0 shows that a user is using version 1.0 of a product called RETSclient. This “signature” is provided with each RETS request and can be used for authentication and/or filtering on the server.

User Class

See User Roles

User Roles

A group of users can be assigned specific user roles. Typically, these roles are used in combination with filtering so a group of users are assigned specific filtering. For example, any RETS user who is an IDX vendor can be assigned the “IDX Vendor” role and a specific filter can be applied to the group rather than to each user specifically.


Mechanism which tracks all changes in content and code. For RETS, Versioning refers to the number associated or applied to the RETS Standard Documents, i.e. 1.5, 1.7, etc which have been voted and approved by the RETS Working Group.


An application programming interface layer between outside applications and MLS data / servers using open data approach of the widely-adopted RESTful (REpresentational State Transfer) technology in use by many industries today


Workgroups are responsible for updating and creating new standards as the need or desire arises. Workgroups are comprised of RESO members who contribute their time on a volunteer basis.

WS (Web Services)

A standardized way of integrating web-based applications using the XML, SOAP, WSDL and UDDI open standards over the Internet protocol backbone.


eXtensible Markup Language. XML is a custom markup language used by many general-purpose applications to exchange information. The tags used within XML are not pre-defined and can be anything that correctly describes the data. Data in this format is easy for a program to consume and use, regardless of the operating system or programming language used. Within RETS, the XML tags are pre-defined which make it easier for a single program to understand data that may come from different RETS servers.