About requirements and design
The requirements describe what is needed and expected to solve a given problem.
The design describes a possible solution to that problem.
The SRS is the System Requirement Specification. So it shall describe the problem space and not the solution.
All kind of diagrams can be used in such a document: plain old data flow diagrams, ...
It's crucial to separate requirements from design.
Requirements are about what the system does, while the design is about how the system does it. You typically need some kind of requirement, whether that's a formal requirement in the sense of IEEE 830 or ISO/IEC/IEEE 29148 or an informal requirement in the sense of a representation of a stakeholder need or ...