4ed - Marked?

[Grug]

While reading some of the leaked 4ed info, I noticed that one of the Swordmaster abilities caused the target to be "Marked". What does that mean? The description just said that the target becomes marked, nothing else.

[Xefas]

A creature who is Marked takes a -2 penalty on all attack rolls against targets that are not the person who Marked them.

A creature can only have one Mark on it at a time.