They each have their own seperate bid. Usualy it takes place on the same day and starts the same day. Also catering is a closed department and they have their own seperate bid. You have full time lead, full time agent, part time agent, full time catering and part time catering. And in some cities they have a full time tower lead bid.
I worked in a small station for 4 years part time.They rarely if ever upgraded to full time unless there was a full time lead position opening (USAirways has no part time lead positons in fleet service). I transfered to a hub and before I unloaded my first aircraft I was upgraded to full time agent and slotted in on a shift. When the first full time bid came up I lost 2 years seniority and I dropped nearly 50 slots for bidding and layoff purposes. Their were agents with 3 years seniority that basically got full time within a few weeks of being hired that moved ahead of me even though I had 4 years with the company in fleet service. I have been thru 3 mergers (PSA, Piedmont, America West) and if anybody thinks there will not be layoffs they are fooling themselves. Seniority is everything in a merger and now is the time to get it corrected. Not after the merger !