Pretty much a system-wide seniority list.
If there is a captain slot open on the 737, everyone has the contractural right to bid on the open captain slot, including the most junior FO on the list up to the most senior captain on the 777.
The pilot with the most seniority is awarded the 737 captain position.
Then once you're captain, when you bid for a schedule, your award depends on your relative seniority to the other 737 captains in your base.
Now I'm confused!