How does the ticket system in Operations work?

There are lot of similarities with Rush. Like in rush, the attackers are given a limited number of reinforcement tickets. In Operations by default it's 250 for 64 players, 200 for 40 players (150 for both prior to the fall update). When an attacker dies, a ticket is lost. When Medics revive a teammate, a ticket is refunded.

When tickets are at zero, game continues as long as there isn't a flag fully in defenders possession (so for example either both are disputed or one is taken by attackers, other is disputed).

Unlike Rush however, in Operations advancing to next set of objectives does not fully reset the ticket count, instead you get additional 50 tickets (30 prior to patch). There is however an opportunity to gain some additional tickets. Retreating defenders are marked and killing each of them grants attackers 3 tickets (2 prior to the fall update)