segtax (Segment Taxonomy Code)

Segtax (segment taxonomy code) is a 16-bit field in OpenRTB 2.6's user.data[].segment[] envelope that identifies which audience taxonomy a segment ID belongs to. The same numeric segment ID (e.g. "10") means different things in different taxonomies; the segtax disambiguates.

The published segtax codes as of 2026 include: segtax=1 (IAB Content Taxonomy 1.0), segtax=2 (IAB Content Taxonomy 2.0), segtax=4 (IAB Audience Taxonomy 1.1), segtax=5 (IAB Content Taxonomy 2.2), segtax=6 (IAB Content Taxonomy 3.0). Custom or vendor-specific taxonomies use values ≥500 — segtax=500 is reserved for Permutive, segtax=600 is reserved for Trillboards, etc. The full registry is at iabtechlab.com.

Segtax was introduced in OpenRTB 2.5 as the answer to a real-world problem: bid requests routinely carry segments from multiple data providers, each using their own ID space. Without segtax, a DSP had to either negotiate a single taxonomy with every supply source or risk mis-targeting. With segtax, each segment is self-describing.

Trillboards uses segtax=4 for everything mappable to IAB AT 1.1 plus a Trillboards-namespace segtax=600 for CV-unique signals (cohort composition, attire archetype, activity macro) that have no IAB equivalent. Downstream consumers can choose to honor only IAB segtaxes for cross-vendor portability, or include the Trillboards namespace for higher-fidelity DOOH targeting.