A Business Requirements Document (BRD) is a formal contract between the customer and the organization for a product.
The difference between BRD and SRS are as follows:
BRD
It is a high-level functional specification of the software.
It is a formal document to describe the requirement provided by the client (written, verbal)
The Business Analyst creates it after their direct interaction with the clients
It is derived based on the requirements and client interaction
SRS
It is a high level functional and technical specification of the software
It describes the functional and non-functional requirements of the software to be developed
The System Architect creates it as it needs technical expertise. Though sometimes Bas too can create it.
It is derived from the BRS
