InstantAction is a nickname in the Crysis series given to the infamous "DeathMatch" or "Free For All" type of game mode.
It's rules are simple - every player is an enemy, the player with the highest kill count/score at the end of the match is the winner.
Here you'll find the example InstantAction.xml script that is provided with the SDK. Below it you'll find information on what each of these sections mean.
<Mode name='InstantAction'>
<StatsRecording class='StandardStatsRecording' />
<Spawning class='MPRSSpawning' teamGame='0' teamSpawnsType='None' usingLua='0' />
<Scoring class='StandardScoring'>
<Player>
<Event type='PlayerKill' points='200' xp='200' />
</Player>
</Scoring>
<ScoreRewards enabled='1' />
<!-- NO ASSIST scoring in instant action -->
<!-- <AssistScoring class='AssistScoring' maxTimeBetweenAttackAndKillForAssist='7000' maxAssistScore='99' assistScoreMultiplier='0.5'/> -->
<PlayerStats class='StandardPlayerStats' />
<State class='StandardState'>
<StartStrings startMatch='ui_msg_ia_matchstart' />
</State>
<VictoryConditions class='StandardVictoryConditionsPlayer' killsAsScore='1' />
<PlayerSetup class='StandardSetup' />
<DamageHandling class='MPDamageHandling'>
<Table path='Scripts/GameRules/MPDamageTable.xml' />
</DamageHandling>
<ActorAction class='MPActorAction' />
<Spectator class='MPSpectator' eatsActorActions='1' enableFree='0' enableFollow='0' enableKiller='1' timeFromDeathTillAutoSpectate='5000' />
</Mode>
Script.ReloadScript("scripts/gamerules/GameRulesUtils.lua");
InstantAction= {};
GameRulesSetStandardFuncs(InstantAction);