How to read OpenRTB top-level control fields

Purpose

This document explains the top-level control fields in an OpenRTB BidRequest that shape auction behavior, response timing, buyer restrictions, and blocking policy. Unlike site, app, device, or user, these fields are not primarily about context. They are about how the transaction itself should be controlled.

Key Takeaways

• test, at, and tmax define the runtime behavior and time limits of the auction.
• wseat, bseat, bcat, badv, and bapp define who may bid and what kinds of ads should be restricted.
• cur, wlang, and allimps help describe currency, language, and packaging scope.
• These fields are easy to overlook, but they matter a great deal in real exchange operations.

One-Page View

1. Why these fields matter

• imp, site, and app describe what is being sold.
• Top-level control fields describe under what rules that opportunity is being transacted.
• If they are absent or ambiguous, bidders often react conservatively or skip the opportunity altogether.

2. Auction control fields

Field	Spec Status	Practical Importance	Practical Interpretation
test	Optional, default 0	Medium	Distinguishes test traffic from billable production traffic.
at	Optional, default 2	Very high	Defines the auction type. It directly affects pricing strategy and billing interpretation.
tmax	Optional	Very high	Sets the maximum response time allowed for bidders. It directly influences timeout behavior and fill rate.

Implementation Notes

• test is not just a debug flag. It is also important for separating operational reporting from test traffic.
• at is essential for interpreting whether the market is behaving like first-price or second-price-plus logic.
• tmax that is too short hurts response rates. tmax that is too long can hurt user experience and render timing.

3. Buyer seat restriction fields

Field	Spec Status	Practical Importance	Practical Interpretation
wseat	Optional	High	Allows only specific buyer seats to bid.
bseat	Optional	High	Blocks specific buyer seats from bidding.

Implementation Notes

• wseat and bseat are generally not used together.
• These fields only make sense when buyer seat identity is coordinated in advance.
• They often connect to PMP, agency seat, and reseller seat control.

4. Blocking policy fields

Field	Spec Status	Practical Importance	Practical Interpretation
bcat	Optional	High	Blocks advertiser categories.
badv	Optional	High	Blocks advertiser domains.
bapp	Optional	Medium to high	Blocks application identifiers and is especially relevant in app and CTV contexts.

Implementation Notes

• bcat should be read together with the category taxonomy in use. In 2.6, cattax becomes part of the interpretation.
• badv connects domain-based advertiser policy to brand safety and inventory controls.
• bapp depends on correct app store ID and bundle or package conventions.

5. Supporting control fields

Field	Spec Status	Practical Importance	Practical Interpretation
allimps	Optional, default 0	Medium	Indicates whether the imp[] array represents all relevant opportunities in the current context. Important for road-blocking interpretation.
cur	Optional	Medium to high	Declares allowed bid currencies. Important when an exchange accepts multiple currencies.
wlang	Optional	Medium	Restricts the allowed creative language set. Important when language policy matters.

Implementation Notes

• allimps does not appear in every implementation, but it can matter in multi-slot and pod-style contexts.
• cur becomes much more important when the exchange accepts multiple currencies.
• In OpenRTB 2.6, wlangb also exists as a more modern language encoding path alongside wlang.

6. How to think about these fields

• These fields are less about object modeling and more about transaction policy.
• Their presence may look secondary, but misinterpreting them can have outsized operational consequences.
• In practice, at, tmax, bcat, and badv often have immediate effects on bidder behavior and policy compliance.

Implementation Notes

• It is useful to group these fields in logs and configuration under auction policy, buyer policy, and content policy.
• test, at, and tmax should be visible in request-level logs.
• bcat, badv, and bapp should be traceable back to their originating supply-side policy configuration.

Prerequisite Concept

• What Is OpenRTB

Next Documents

• How to Read site, app, and imp
• OpenRTB 2.6 Required and Recommended Fields at a Glance

Related Documents

• What OpenRTB 3.0 aimed for and what returned in 2.6

Official References

• OpenRTB 2.6 PDF
• OpenRTB 2.5 FINAL PDF
• IAB Tech Lab OpenRTB Standard