Overall schedule bidding occurs in two "rounds", round 1, and round 2 (often times stylized as RD1/RD2). Round one will have X amount of lines for pilots of a certain base and seat (the exact number depends on the month). If you are awarded a round one line, you would eligible to "SAP" (to take part in the schedule adjustment period). Captains go first, FOs after them.

SAP covers a 24-hour period where any and all trade requests are processed first-come, first-served. Biggest rules being that you can't drop below 65 credit hours for that month, and you can't do anything that would create contract/legal violations (duty issues, etc). So if you are awarded a 4 day trip that's not commutable and you commute to base, you can swap it for any other trip that is legal to put on your schedule, even if it means gaining days off, or losing credit (respecting the 65 rule).

What this tends to mean for people, is that they can mostly build the schedule that they want, regardless of what they were awarded. Some people get exactly what they want right in the initial award, and others have been known to move their entire month around, not keeping a single original trip they were awarded.

The benefit of all of this is that you don't have to worry about whether or not that day has enough staffing. If the trade is legal, it should go through. This is because, whatever is left over after this period of trades, now gets built up into more lines, that are then awarded during the round 2 bid. Theoretically if people drop tons of flying in round one, there would be more round two lines, and vice versa.