I am familiar with writing Software Requirements Specification, but not quite sure how to write the requirements for a specific module.Let's say I need to develop a Reservation System and  there will be a module to perform validation. Now, an SRS is already available for the entire system, but prior to designing(API Specification and Class Diagrams) the Validation module, what would be the format of writing Requirements for this module.
How to write Requirements for a specific module
requirements;specifications;modules
A specific model shouldn't require more requirements, a module should complete or be part of the solution to a subset of overall requirements for the project. If you find that a module doesn't work towards fulfilling project requirements, then it,s either unnecessary or your requirements are lacking. Creating modules is part of the design process, requirements should already have been set, if you are trying to make specific requirements for a specific module then you are doing it wrong.