Secondary lines are a block or multiple blocks of days (this month, my base was mostly sets of two blocks) in a month that are blank slates when they're awarded. The contractual requirement is that you check your schedule 2 days prior to the start of your block (between 17&18z) to see what they've built for you. If it's blank, you check again the following day until your block is up or they've added some flying to your schedule, at which point, you aren't required to leave home until the following day. There are some contractual protections about how long they can leave you sitting out in the field and when they must return you to base/cut you loose. Essentially it's a long call reserve with no requirement to sit around and wait for the phone to ring and protections against being extended.