To specify in an interface model that the implementation behind the interface will at a certain moment perform an action requiring a response to be sent over the interface the keyword inevitable or optional can be used.
Inevitable means that when no other events occur, this event will occur. Optional means that the event may or may not occur.
Note that an Inevitable event is not guaranteed to occur always. If there are other (optional or inevitable) events that can occur, the inevitable event may or may not occur. So it is only inevitable in the absence of other events.
The difference between the two is how the event is handled by the model verification. For inevitable events, the verification only checks the cases where the event does occur, i.e. it assumes that the event will eventually occur if no other events occur. For optional events, the model verification also checks the cases where the event never happens (which could be a cause of deadlock).
If both optional and inevitable events are used in the same state, it is only guaranteed that one of the two events will occur, but it is not guaranteed that the inevitable event will always occur. In this situation there is no difference between inevitable and optional anymore since the occurance of any of the events is a correct scenario. In fact the classification of 'inevitable' may be interpreted as property of the block of events.