Lines are awarded to flight attendants by seniority. This means who ever was the senior of all the flight attendants who bid for a specific line number will get that line. Of course you bid for more than one line.

In the airline industry, seniority is everything—it is the primary factor that dictates a flight attendant's quality of life. Seniority is based on your "hire date" (specifically, the date you completed initial training). Those who have been with the company the longest are at the top of the list and get first pick of everything. This includes bidding for monthly schedules, choosing specific routes (e.g., "senior" flight attendants often take long-haul international flights to Paris, while "junior" ones work multi-leg domestic "turns"), and selecting vacation dates. Seniority also determines who gets "reserve" duty (being on-call) versus having a "line" (a set schedule). Perhaps most importantly, seniority provides job security; in the event of airline furloughs or layoffs, the process happens in "reverse seniority," meaning the last people hired are the first to be let go. Because of this, flight attendants rarely switch airlines, as doing so would mean "losing their number" and starting back at the very bottom of the list at a new company, regardless of how many years of experience they have elsewhere.