Business Specifications are documents, the purpose of which (note this node has an IT/IS focus) is to;

• Understand where business rules fit with business strategies, to ensure continuing business alignment of the project
• Communicate between business and IT professionals, to avoid misunderstandings and conflicts
• Develop a system solution based on a business model and business rules, to avoid costly rewrites and delays
• Enhance your IT methodology, to capture and develop requirements more effectively; and
• Improve business and systems acceptance using business rules as a basis for test plans

What all of the above jargon boils down to is the fact that, while there are many rules and tools for creating useful business specifications, most of these are (while useful), often merely propaganda.

There is no set way to do a business specification, but things like the traditional Systems Development Life Cycle (SDLC), Requirements Specifications, Structured Case Analysis and Management Theory and all of its variants are often used, for example.

What rules and tools that will be useful depends on exactly what type of business the specification is for.