Model requirements as:

Cause-Effect-graphs

A Cause-Effect-Graph (CEG) captures logical dependencies between different variables mentioned in a requirement. For example in the requirement: “If the payment type is wire transfer, and the button “Add Reference Number” is activated”, there is a dependency between the variables “payment type” and “button activation”.   A CEG captures such dependencies graphically. A CEG consists of cause nodes, effect nodes and conjunctive AND, OR and NOT nodes.

Below is an example, how a requirement can be represented as cause-effect diagram.
The requirement is:

The contract management system approves a new contract if no automatic checking rule raises an alarm and/or the contract is manually approved by an accountant and the responsible agent.

For this requirement a suitable CEG is shown below.

Activity Diagrams