Responding to an active threat (shooter) will vary based upon the attention of the threat, your readiness, and your carry status (whether or not you have a firearm and if it's open or concealed carried). In the event a threat is actively engaging targets, you must take action. RUN, HIDE, FIGHT. There are endless scenarios and possible outcomes but in this "react to contact" drill we'll highlight the 4 best courses of action (COAs) for the below scenario. Keep in mind this is a generalized threat response and it can be applied to multiple scenarios.
ACTIVE TARGET - The shooter sees you and determines you are a threat. You are openly carrying/holding a weapon, you are visibly dialing 911, or you are otherwise making yourself the shooter's primary target.
COA: Engage, move from line of sight (LoS) across concealment (a barrier that masks you but doesn't stop bullets) to cover (a barrier that stops bullets), reenage.
KINETIC TARGET - The shooter may see you but they are not actively targeting you due to the perception you are not a current threat to their goal. You are armed but carrying concealed and not wearing clothing that would identify you as a danger to the shooter.
COA: Move from LoS across concealment, draw weapon, engage if necessary while moving to cover, then engage upon cover.
PASSIVE TARGET - The shooter may see you but they are not actively targeting you due to the perception you are not a current threat to their goal. You are an unarmed bystander.
COA: Move across concealment, put cover between you and the shooter, exit the area or hide (shelter in place).
DUEL ACTIVE TARGET - Same as an active target but in this scenario you have an armed friendly.
COA: The closest friendly should immediately engage and provide suppressive fire while the other moves to cover, bounding as necessary, flanks and engages from shooter's vulnerable side. Both should continue engaging from first encounter until threats are stopped.