A functional requirement document defines the functionality of a system or one of its subsystems. It also depends upon the type of software, expected users and the type of system where the software is used.
Functional user requirements may be high-level statements of what the system should do but functional system requirements should also describe clearly about the system services in detail.
The following are the key fields, which should be part of the functional requirements specifications document:
Purpose of the Document
Data and Integration
Data Migration & Conversion