The card Haunted Plate Mail tells "Activate this ability only if you control no creatures."
I want to make sure I understood it right: Only if you control no Creatures on the Battlefield?
Board & Card Games Stack Exchange is a question and answer site for people who like playing board games, designing board games or modifying the rules of existing board games. It only takes a minute to sign up.Sign up to join this community
The rules for permanents, specifically 110.2, says
A permanent's owner is the same as the owner of the card that represents it (unless it's a token; see rule 110.5a). A permanent's controller is, by default, the player under whose control it entered the battlefield. Every permanent has a controller.
Only permanents have controllers, and 110.1 says "A permanent is a card or token on the battlefield," so yes, "creatures you control" means "creatures on the battlefield that you control."
In addition, it's worth mentioning that rule 109.2 says
If a spell or ability uses a description of an object that includes a card type or subtype, but doesn't include the word "card," "spell," "source," or "scheme," it means a permanent of that card type or subtype on the battlefield.
Since Haunted Plate Mail just says "creatures," it is referring to creature permanents on the battlefield.