Systems specified in SDL behave according to the stimuli received from the external world. This external world is called the environment of the system being specified.
It is assumed that there are one or more process instances in the environment, and therefore signals flowing from the environment towards the system have associated identities of these process instances. These processes have PId values which are distinguishable from any of the PId values within the system (see Annex D, §D.10)
Although the behaviour of the environment is non-deterministic, it is assumed to obey the constraints given by the system specification.