The flare is an ammo type that is specifically used in the flare gun. The round has a familiar look of a shotgun shell but with red paint on the sides. As this is a type of ammo, it has no weight, except in Survival mode.
After retaking the Castle and completing Old Guns to gain access to the armory, 10-13 flares will respawn in a crate along with artillery smoke grenades in the shelves that contain turrets and traps on the right side of the wall.
Needs to be within range of an allied settlement to summon help.
Maximum range is around 3 grid cells on the in-game map.
It is possible to increase the range to around 4 grid cells by aiming the flare gun at about a 45 degree angle directly towards the nearest settlement. This is because the event trigger appears to be the ignited flare's location and not the Sole Survivor at time of check. The summoned Minutemen will come the player character's location though. This is why sometimes it doesn't appear to work, even within sight of a settlement. Basically the flare travels up to 1 grid cell distance.
Summons between 3-6 Level 1 Minutemen characters.
Can be used unlimited times in any in-game hours.
On firing the flare for aid, if in range, they will send up another flare in response to signal that they are incoming.