Regarding software requirements, there are several scenarios, generally that should be illustrated, considering different conditions while using the software. When we use UML, two types of diagrams comes in place, one is 'Activity diagram' which preferably can be used from top level perspective or the 'Interaction diagrams' (Sequence/Collaboration diagrams) in that case.
On the other hand 'Screen Mockups' or 'Prototypes' are used to illustrate the snaps of the real software, well but mock-up are static in that sense, and are not able to give the idea about the accurate and dynamic behavior of the software.
I believe, an alternative visual language should be developed so that we can capture both requirements with same set of diagrams and notations and thus when both requirements will be full filled it will provide us a faster, clear and specific way to elicit, discover, understand and update the system requirements.