How do Flags work?
Flags are markers you can set in the game, to later check whether they’re on or off. Some flags are Temporary Flags, meaning they turn off after every chapter, while others are Global Flags, meaning they remember their setting throughout the playthrough.
An event can have a thing called a Completion Flag. This specifies a particular flag for the game to turn on when that event runs, as an indicator that the event has happened. If the Completion Flag is already on, the event cannot run, as the game sees it as already completed.
How do I make zone-based reinforcements?
Zone-based reinforcement triggers have two steps, since you usually want the reinforcements to appear at the start of the next player phase, not immediately upon entering the zone.
The first part is the actual trigger, a Range Event set for when a player unit enters. This event needs a Completion Flag, let’s say Temporary Flag 8, so that it only happens the first time a unit enters the area, instead of happening every time a unit enters the area.
The second part is a turn-based event to summon the reinforcements at the start of player phase. This event is set to run every turn – start turn 1, end turn 255=infinite. However, this event also has a Completion Flag, let’s say Temporary Flag 9. By itself, this would mean that the event would run only on turn 1, and then not run after that because its Completion Flag is on. But instead, what you can do is have the Completion Flag be turned on manually in the Chapter Start Event, such that the reinforcement event never runs… until you turn its Completion Flag off manually in the Range Event, after which it will run once on the following turn!