Evrim:

Regarding system behavioral requirements:

To properly structure (i.e., organize) requirements at the big picture level on larger scale projects, you need to use a non-sequential based process/functional modeling technique.  This is because in these situations there is no definable sequence.  It is not possiblele to use a sequence oriented technique to model a non-sequential situation. 

Data flow diagrams, user stories, and use cases are non-sequential, however, only data flow diagrams have an integration mechanism.

At the small picture level, you can model using a sequenced based modeling technique such as BPMN (or other BPM technique) and activity diagrams.

Tony