How to write Requirements for a specific module

Question

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.