burl

burl is the billing notice URL in an OpenRTB bid: the address the exchange calls when a winning bid becomes billable under the exchange's rules, which tells the buyer to count the spend.

Billing notices arrived in OpenRTB 2.5. A billing event is the point at which a publisher charges the exchange and the exchange charges its buyer. For a DSP, the call means it can add the spend and deduct it from the campaign budget (IAB Tech Lab, OpenRTB 2.x implementation notes, section 7.8).

The IAB Tech Lab's best practices are specific. The exchange should fire burl from its own servers, as close as possible to the moment it books revenue. It should not hand the job to a client-side pixel, although a client event may trigger it. If the call gets a response other than HTTP 200 or 204, the caller should retry, for example every 10 seconds for a minute. The receiver should make its endpoint idempotent so a retry is not counted twice. For VAST video and audio, the VAST impression event is the official billable signal.

Example: A mobile app renders a winning interstitial. The SDK reports the render to the exchange, which fires the DSP's burl from its server with ${AUCTION_PRICE} filled in. The first call times out, the retry succeeds 10 seconds later, and the DSP's idempotent endpoint records one billable impression.

Related terms: nurl, loss notification, bid response, OpenRTB, frequency capping.

Authoritative source: IAB Tech Lab, OpenRTB 2.x implementation notes, section 7.8 "Counting Billable Events and Tracked Ads" (accessed 9 November 2026).

Read the full guide: OpenRTB 2.6 explained: bid request anatomy, field by field.