It is sometimes (if rarely) necessary to specify a primitive process which terminates. Consequently, a local process STOP is predefined which engages in no further actions. In LTS terms, STOP defines a state with no outgoing transitions. The example is a process which does some init action and then terminates.
STARTUP = (init -> STOP).
It should be noted that STOP means that the primitive process within which it is declared takes no further actions. It does not mean that the global system composed of potentially many primitive processes can take no further actions (which would be a deadlock),