A formal record accompanying a set of documents or models sent from one party to another, listing what was sent, when, and why. A transmittal is the paper trail of information exchange - it names the sender, recipient, purpose (for information, for review, for construction), revision numbers, and a list of every file included, so both sides have a shared, auditable record of what was issued and when. On BIM projects it's usually generated automatically by the Common Data Environment when a set of documents changes CDE state.