Business application header (BAH)

The Business Application Header is a header that has been defined by the ISO 20022 community, that can form part of an ISO 20022 business message. Specifically, the BAH is an ISO20022 message definition (head.001.001.0x) which can be combined with any other ISO20022 message definition to form a business message.
 
It gathers together, in one place, data about the message, such as which organisation has sent the business message, which organisation should be receiving it, the identity of the message itself, a reference for the message and so on.  
The purpose of the BAH is to provide a consistent and predictable way for this data to be conveyed with the message, regardless of implementation factors such as the choice of network. This does not prevent such data being conveyed either within the ISO 20022 message definition itself, or as part of a network header.

You can find the documentation for the BAH in the catalogue of messages.