I'll be specific as it relates to Mesaba.
Let's look at MEM Saab F/O bid package for February.
There are 47 Saab F/Os in MEM. For the month of February, 39 of them are eligible to bid and are ranked by seniority number. (8 are not eligible for various reasons).
For February MEM Saab F/O bid pack has 30 hard lines, 5 reserve lines and 4build-up/reserve.
The lines are ranked by weekends off and days off and total credit. Typically, you can figure that of the 39 eligible bidders, the top 30 will get a hard line in February, 5 will get a reserve line, and the remaining 4 will get a build-up/reserve.
In any given month, a more senior pilot may elect to bid Reserve to be at home and get work done on the house or something. Reserve call outs go in reverse seniority, so a senior pilot on reserve will be used last. In this case, if you are bidding 31 or 32, you could from month to month hold a line and then not, depending on the pilots above you opting reserve for whatever reason. If you are in the bottom 4 or 5, you can plan on not holding a line.