IDX and MLS data glossary
The words behind MLS data feeds and IDX websites, defined the way they are actually used.
IDX
IDX (Internet Data Exchange) is the MLS policy that lets a broker display every participating broker's active listings on their own website, not just their own inventory. It is why an agent site can show the full market. IDX covers on-market listings only and comes with display rules: attribution to the listing agent and office, an MLS disclaimer, and a refresh at least every 12 hours.
MLS
An MLS (Multiple Listing Service) is the broker-owned database where listings are entered first, with rules on accuracy and cooperation. Portals like Zillow receive copies through syndication, which is why portal data can lag or disagree. IDX is how a broker's own website shows MLS listings directly, on the same footing as the big portals.
CRMLS
California Regional MLS (CRMLS) is the largest MLS in the United States, covering most of Southern California, including Los Angeles, Orange, Riverside, and San Bernardino counties, plus many associations beyond. If you are an agent in the LA metro or the Inland Empire, your listings almost certainly live in CRMLS, and any IDX website you run needs a CRMLS-authorized data feed.
VOW
IDX shows on-market listings to anyone visiting a website. A VOW (Virtual Office Website) can show more, including sold history, but only to consumers who register and agree to terms, because a VOW legally stands in for working with a brokerage. Most agent websites need IDX; a VOW is for portals that gate content behind an account.
IDX Plus
A standard IDX feed is on-market only. CRMLS offers an expanded feed, often called IDX Plus, that adds recent sold data with the selling agent and office. Whether solds can appear on a public page depends on the feed type your vendor is licensed for, so ask the vendor which feed backs your site before promising a sold-homes page.
RESO Web API
The RESO Web API is the modern standard for MLS data feeds: JSON over OData with standardized field names (the RESO Data Dictionary), replacing the old RETS protocol. When a vendor says they deliver MLS data by API, RESO Web API is the upstream format they are reading; what your website consumes is whatever the vendor exposes downstream.
RETS
Yes. RETS (Real Estate Transaction Standard) was the 2000s-era MLS feed protocol, and MLSs have retired it in favor of the RESO Web API. CRMLS distributes feeds through Trestle as RESO Web API only. If a tool or plugin still asks for RETS credentials, it is built on a protocol your MLS no longer supports.
Trestle
Trestle is Cotality's (formerly CoreLogic's) data distribution platform. CRMLS does not hand out feeds directly; every CRMLS data license flows through Trestle, where the vendor signs up as a Technology Provider, describes the product, and e-signs the Data License Agreement. For an agent, this is invisible; it is the vendor's paperwork.
Data License Agreement (DLA)
The Data License Agreement (DLA) is the contract between an MLS and a technology vendor that licenses the listing feed. It names the specific product, its URL, and its use, and the MLS can decline a vendor whose product description does not clearly serve MLS members. The DLA sits on the vendor; the agent's separate step is the IDX authorization form.
CRMLS IDX Request Form
A CRMLS agent gets IDX on a website by filing the CRMLS IDX Request Form: the agent and their broker of record sign it, naming the website URL and the IDX vendor, and it goes to Licensing@crmls.org. No form, no live data, no matter which vendor you pick. A good vendor pre-fills the form and files it for you.
Broker of record
Yes. IDX authorization belongs to the brokerage, so the broker of record signs the IDX request alongside the agent. This is an MLS rule, not a vendor policy: listings are licensed to brokers, and an agent's website displays them under the broker's participation.
IDX vendor
An IDX vendor, also called an IDX provider or MLS data vendor, is a company licensed by an MLS to deliver its listing feed into member websites. The vendor signs the data license, keeps the feed synced, and enforces the display rules; the agent or broker authorizes the vendor for their site. HomesIDX is a CRMLS IDX vendor that delivers the feed as a JSON API and a hosted search page.
Attribution
CRMLS requires the listing agent and listing office to be identified directly adjacent to the price, beds/baths, square footage, or photo, in a font no smaller or lighter than the description, labeled 'Listing Agent' and 'Listing Office' (the phrase 'Courtesy of' is non-compliant). Any contact button must say which agent responds. These rules are why compliant IDX is harder than 'just show the data'.
IDXContactInfo
IDXContactInfo is a field CRMLS includes in its feed telling displays how to contact the listing broker, and CRMLS rules require it to be shown as delivered on IDX displays. It is one of the fields a compliant vendor returns with every listing so a website cannot accidentally omit it.
Refresh interval
NAR policy requires IDX data be refreshed at least every 12 hours, and off-market listings must come down within that window. In practice, buyers notice stale data long before 12 hours, so a good feed replicates incrementally every 15 minutes or so and reconciles daily.
Participant and subscriber
A participant is the broker who holds MLS membership; subscribers are the agents licensed under that broker. IDX rights flow from the participant, which is why every IDX site, even a solo agent's, is authorized by the broker of record. MLS data vendors may only serve participants and subscribers, not the general public.
OData
OData is the query protocol the RESO Web API uses: standard URL syntax for filtering ($filter), paging ($top, @odata.nextLink), and metadata ($metadata). A developer pulling raw Trestle data writes OData queries; an agent using an IDX vendor's API never sees it, because the vendor translates it into plain search parameters.
MLS disclaimer
Every IDX display must carry the MLS's disclaimer and copyright line, a data-last-updated timestamp, and the Equal Housing Opportunity logo, alongside per-listing attribution. The exact disclaimer text comes with the feed; a compliant vendor returns it in every API response so it ships with the data instead of being pasted once and forgotten.
Hosted search page
Yes. A hosted search page is a ready-made MLS search and listing-detail experience the vendor runs for you, on your domain or a subdomain, already laid out to pass display rules. It is the fastest route to compliant IDX: broker authorization plus a DNS record, no development work.
Pocket listing
Common reasons a listing is missing from IDX sites: the seller signed an opt-out excluding internet display, the listing is an office exclusive not entered in the MLS (a pocket listing), the status moved off-market, or the display's feed has not refreshed yet. If a listing is in the MLS with internet display on, it appears in every IDX feed automatically.