Would you include an XML schema for a new file format in a software specification document or would you consider this too specific? Or would it be already too specific to require XML at this point?
Should an XML Schema be specified in a software specification?
specifications
If the schema already exists, then I don't see why you wouldn't want to specify that your system must use a particular schema (and version of that schema) for data storage or transfer. This sounds like a non-functional requirement to me, specifically what Karl Wiegers refers to as either a Software Interface Requirement or a Communications Interface Requirement.However, if the schema is to be constructed, then the requirement itself should not even specify that XML is used to store and transfer data, but the constraints should be documented elsewhere in the requirements document(s). In this case, the requirement should be written to inform the developers of what must be accomplished, giving them as much room to make design decisions as possible. Again, the SRS template provided by Karl Wiegers has sections titled Operating Environment and Design and Implementation Constraints where you can specify various considerations that developers must be aware of when making the system.