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Business Requirement Documents are written to define the requirements of a business process or a system that needs to support a business process.  For purposes of contrasting the Business Requirement Document (BRD) and the Functional Specification Document (FSD), the description of the BRD that follows is written in terms of preparing a BRD for a system. The exact scope of a BRD and FSD vary from company to company.  However, the two documents are typically defined as follows:

The BRD contains the business requirements that are to be met and fulfilled by the system under development.  These requirements specify "what" the system must do in order to fulfill the requirements of the business.  They often take the form of "The system shall..."  Each requirement, or group of similar requirements, is typically accompanied by a business rationale. The business rationale explains "why" the business requirement is necessary.  This is often important later if analysts or developers have questions regarding the purpose or validity of the requirement.  The rationale can be used to support the need for the business requirement or clarify ambiguous language by providing a context for the requirement. In addition to a rationale, constraints can be provided for each requirement along with other supporting reference material.

In contrast, the FSD defines "how" the system will accomplish the requirements by outlining the functionality and features that will be supported by the system.  Ideally, the functionality of the system will be described in logical terms so that the FSD is technology and platform independent. This gives the architects and developers more freedom in making development and design decisions about the physical design of the system.  Inevitably, however, some things have to be explained in physical terms.  The User Interface is one such example.  Many FSDs include screen mockups or wireframes for communicating the layout and design of the system screens.