The client sends a TimeStampReq with a messageImprint (hash algorithm and digest, usually of the signature value) and optionally a nonce; the TSA returns a CMS SignedData whose content is a TSTInfo with policy, serial number, genTime, accuracy and the imprint (RFC 3161, updated by RFC 5816 for SHA-2 certificate identifiers). The TSA's certificate carries the critical timeStamping EKU.
Code signing (Authenticode counter-signatures, Java jarsigner -tsa), PDF signatures (PAdES B-T and higher), CMS and S/MIME archives, and Sigstore (as an alternative to Rekor timestamps) use it. Without a timestamp, a signature becomes unverifiable when the signing certificate expires; with one, verifiers check the certificate's validity at the timestamped moment. The timestamp token inspector decodes tokens locally.