413.1. Buffing is the action of placing a Buff counter on a Unit . See rule 701. Buffs for more information.
413.1.a. A Buff is also an object.
413.1.b. To Buff a unit, place a Buff Counter on it if it does not have one already
413.1.b.1. If the unit already has a Buff Counter on it, it does not get another one.
413.1.c. Units with Buff Counters can still be chosen for actions that Buff units, but will not be Buffed as part of the execution. Example: A spell reads "Buff a unit. Then, if it was buffed this way, draw a card." It will place a buff counter on a unit that has no buff counter, and then its controller will draw a card. However, if the player chooses a unit with a buff counter on it already, then the card will not be drawn.