What is the SupplyChain object (schain)?
The SupplyChain object, or schain, is a field in a programmatic bid request that lists every company that handled the impression on its way from the publisher to the buyer, with each seller's account ID. Combined with ads.txt and sellers.json, it lets buyers verify each hop and spot unauthorized or unnecessary intermediaries.
The supplyChain object is an OpenRTB extension from the IAB Tech Lab. Each node records the ad system, seller ID and request ID. A "complete" chain starts with the publisher's own account; an incomplete one means some hops are unknown.
Buyers use it to favour short, direct paths and to block chains that include sellers not listed in the publisher's ads.txt. It is only as trustworthy as the sellers filling it in, which is why ads.cert signing was proposed to make the chain tamper-evident.