Business Application Header (BAH)
The standard envelope in front of every ISO 20022 message, stating who sent it, to whom, and what is inside.
Formally it is head.001. The header carries the sender and receiver, usually as BICs, a unique message reference, the message definition and version that follows, such as pacs.008.001.08, and the creation time. A related-message field is used when a message is forwarded or copied, so the chain can be traced.
The idea is to keep routing apart from business content. A system that only needs to pass a message on can read the header instead of parsing hundreds of elements in the payment itself. In the MT world the numbered header blocks {1:} and {2:} did the same job. Under CBPR+ the BAH is mandatory, and a striking share of early rejections during the migration came from the header rather than the payment.