A mark is a tactical designation placed on a foe, singling them out for attention.
Only one mark can affect a creature at a time. If a creature becomes marked again, the new mark replaces the old one, regardless of the source. A mark remains in effect until its duration expires, you end it as a free action, or the marked creature is defeated.
A creature currently affected by a mark is referred to as “the mark.” Allies can clearly perceive which creatures are marked and by whom, allowing them to coordinate attacks and effects accordingly.