Shorthaul bidline works vaguely like the following (numbers are just guesswork and not precise):
Say you have 250 FOs on a fleet. Pre Ops produce 200 complete rosters ( trip lines), 20 reserve lines (month of standby) and 30 are whats called 'blind lines' with no work on them. The lines are all published at stage 1 and you bid for them in seniority. The goal is to achieve a certain level of work called CAP. Some lines are at CAP, some slightly above, others slightly or much below CAP. Any pre-allocated duties, eg trips carrying in from previous months, sim checks,line checks, etc will take precendence over any conflicting trip and clash it off your line. All the clashed out trips drop to stage 2.
At stage 2 all the remaining trips are published and you bid for them in seniority order to pick up enough work to take you to CAP. If you go over CAP the credit goes into a personal bank. If you go under CAP the deficit is taken from your bank or if you have no bank they dock your pay (first 6 months you can have a bank overdraft whilst you build your bank up). If you want to work a bit less you can bid below CAP and take the financial hit. If you bid lower than CAP -15 you are 'wiped' and assigned a blind line. If you bid below CAP but above wiping level you can be assigned extra work under certain conditions.
If you don't have the seniority for a trip line or your don't bid you get a blind line. This is where the company build a line for you from all the trips left over after stage 2. You are credit protected, which means you are considered to have met CAP even if they actually roster you less work. Expect to work every weekend.
Reserve comes into play three months after your final check. Reserve is allocated in a points system and new entrants join with fleet average points. He who has least points is most vulnerable to reserve. Reserve will be a continuous block of 28 days with 7 days off (usually a block of 2 and a block of 5), with 21 days available for work. One airport standby, the rest is home standby with 2hrs notice to work, but usually much more. There is little predicatability, so don't plan on having a life during that period.
One other feature worth knowing about is 'Forced Draft'. This is basically compulsory overtime which is assigned in juniority order when the company have botchde their work coverage. Its lucrative but try telling that to the stack of people force drafted on Boxing Day and over New Year last year. If drafts about either take the cash or turn your phone off.