It's how airlines prioritize their non-revenue passengers (non-revs). Delta uses S1, S2, S3, S3a, etc. S1 being high priority standby, (space available). PS is positive space, it's almost as if they gave you an actual ticket. You have a seat assigned to you and everything. Catch is, that paying passengers always have priority. So if the flight is oversold, or an earlier flight gets canceled, it's useless. Other airlines have different ways of doing it but it's still similar.