UID object
A UID holds one user identifier inside an EID, together with the type of agent it was observed on. One EID can carry several UIDs from the same source, for example a browser-scoped ID and a person-level ID. Added in OpenRTB 2.6.
How it is used
The exchange should send a UID only when its business agreements and the request's consent signals allow it.
The User identity and EIDs page covers it in more depth.
Fields
Where it sits
Contained in: EID.
All objects
The field reference index lists every OpenRTB 2.x object.