The player with Focus must obey any additional restrictions on which Discretionary Actions may be performed. Example: A player with Focus may not play spells or activate abilities that don't have the Action or Reaction keywords.
§ 313.1.a Core Rules PDF (officieel)
§ 313 At any given time, up to one player has Focus.
§ 313.1 Focus is the permission to take appropriately timed Discretionary Actions when the turn is in a Showdown Open State. See rule 307. States of the Turn for more information.
