When my son worked for Comair and we flew standby a few times, S1 were the employees and their designated companion. S2 was the parent or if the employee did not want to use up their limited S1 points, S3 was the Buddy Pass--the free flight passes that each employee was given for friends and family. Don't know about S4. They were boarded in that priority--1,2,3. As 2s we got on every time--but mostly had to sit in the coach. I flew to Spain once in Biz.