Tag, also known as tag-in/tag-out or tag in/tag out, is the action of a character replacing another character in a match. It can be done by tapping the portrait of a tagged-out character, which causes that character to tag in while the current character tags out, unless there is a Special Attack being performed, the current character is being hit by unblocked basic attacks, or is stunned, frozen, or snared. The portraits can still be tapped. This will cause the portrait to blink, indicating that character will be tagged in on the next possible moment. This cannot be changed or canceled, nor can it be done while tag is on cooldown.
Tagging out will cleanse most negative status effects that don't prevent you from tagging, such as all forms of damage over time, except Lobo/Bounty Hunter's burn, which snares opponents, and any damage over time effects applied by Catwoman/Arkham Knight, which continue to damage tagged-out opponents.
Tagging in a character will place them right next to the enemy character, regardless of the tagged-out character's original position.
Tagging in/out at certain times can cause certain abilities to 'miss' like Wonder Woman's Shield Toss.
Tagging normally has a 10 second cooldown, which can be reduced by 4 seconds by Hawkgirl/Prime's passive.
Uniquely, the cooldown of tag runs in real-life time, instead of in-universe time, unlike most timers in the game (another rare exception is Countdown, with the difference of Super Moves causes tag cooldown to continue to run in real time while Countdown pauses). This means while the game becomes slow motion during a Combo Ender's swipe, or during a Special Attack, tag cooldown will continue to run as normal.
Whenever tag is on cooldown, the tagged-out teammates's portrait becomes darkened. This visual effect also appears if you are snared or affected by Chaos, but not by other effects that prevent tagging.
Some characters have effects that occur whenever they tag in or out, when the opponent tags in or out, only occurs when they are tagged out, or a combination thereof.
Knock out blows will usually count as tagging for self-tag effects, but not for enemy-tag effects. For example, Bane/Luchador's passive activates when he tags in, and is activated when an opponent deals a knock out blow to his teammate and forces him to tag in; Sinestro's passive activates when his opponent tags out, and is not activated when he deals a knock out blow.
The exception is League of Assassins Set's effect, which "traps" the next opponent upon dealing a knockout blow, and the set effect is truly activated when the "trapped" opponent is knocked out or tags out.