Answered by Robert Halligan
The general concept of state allows for concurrency of state, without limitation. However, the benefits of the use of state at a requirements level are maximized to a very high degree by adopting a policy of only defining states that are mutually exclusive, as a starting point, and only admitting concurrency of state if there is clear benefit in doing so. The practical effect of this policy is about 3% of systems or software subject to requirements specification will have any possibility of concurrency of states defined.
An example of states for a smartphone allowed to be concurrent would be a “SIM-Removed State” and a “Battery-Removed State”. These would be mutually exclusive to all the other states (Off, On, Depleted and Failed), which would also be mutually exclusive amongst themselves.