The docs say:
"Suspending a process instance is helpful, if you want ensure that it is not executed any further."
Yet it is possible to modify a suspended process instance and continue process execution that way. This means, starting before an activity will actually execute activities until a wait state is reached.
It is also possible to cancel activity instances of a suspended process instance. However, this might be desired since it is also possible to delete the entire process instance when suspended.
Expected behavior:
modification that starts activities fails
The reason could be that modification creates new executions and these don't inherit the suspension state from their parents.
Possibly related: The same problem might exist with non-interrupting boundary events/event subprocesses, i.e. it might be possible to trigger such an event for a suspended process instance.
Docs on process instance modification:
http://docs.camunda.org/7.3/guides/user-guide/#process-engine-process-instance-modification