In the card game of Munchkin (or Munchkin Zombie) I've seen people put some cards on the table sideways. Usually, they can't use those cards for some reason like it's not for their class or they exhausted body parts limit.
They do that, so they could trade cards, that they cannot use.
Is it used to distinguish equipped and carried items?
The rulebook that came with the game (in the Polish language) does not cover this kind of behaviour.
Edit: For the base version of the game, there is a new one available online which has this covered. Also, all rulebooks in English include this rule.