It is an abstract representation of a business that provides a simplified look of WHAT the business currently does.
The detailed business requirements document outlines the requirements as they are identified in the subject area (this is described as part of the BPM document) in detail.
Basically list each component, the architecture should be seen as a group of artefacts, each artefact resolves a bit of the proposed solution and architecture. It depicts how something’s parts come together to be a whole solution for the requirements.