Here is the Avaya definition of PSA:
The Personal Station Access (PSA) feature allows multiple users to work at the same voice terminal location at different times. PSA provides capabilities that are
similar to TTI, but for a single station. This feature is available starting with DEFINITY ECS Release 5. Each PSA user must have a Station Security Code (SSC), which includes as many as eight digits. The feature has a pre-administered security feature regarding input entry by the
user. Once the user enters his or her extension at the appropriate time, a “no response” feedback is provided whether or not the entered extension is valid. For an invalid extension, the system simply waits, without esponding, until it reaches a timeout threshold. As such, an unauthorized user does not know that input entry is the cause of the error. The same security feature is in effect whenever the user enters the SSC at the appropriate time.
The dissociate function within PSA allows a user to restrict the features available to a voice terminal. Whenever a terminal is dissociated via PSA, it can be used
only to call an attendant, accept a TTI merge request, or accept a PSA associate request.