Source object
Source describes where a bid request came from before it reached the exchange and which party makes the final sale decision. It sits at BidRequest.source, arrived in OpenRTB 2.5, and since 2.6 carries the supply chain object in Source.schain.
How it is used
Header bidding is the case that motivated it: the exchange runs its own auction, but a wrapper or ad server upstream picks the final winner. Buyers read Source.fd to learn whether winning here settles the sale, Source.tid to recognize one opportunity arriving through several exchanges, and Source.schain to verify every seller on the path. All three are recommended rather than required, so older integrations often leave gaps.
The Supply chain (schain) page covers it in more depth.
Fields
Where it sits
Contained in: BidRequest.
Contains: SupplyChain.
All objects
The field reference index lists every OpenRTB 2.x object.