The red box/diamond is a level restriction needed to use the treasure card. This only applies to pvp and it is not for player vs mobs. This is typically effective in low level pvp as it prevents lower level wizards from using tc cards learned later in the game. It basically making pvp as fair as they can for all wizards involved.
Like a level 25 fire cannot use blazing phoenix tc as that has a level restriction of 30. Or a level 9 wizard from using tower shield tc which has a level 10 restriction. If a spell is learned at level 18 it will be ranked down to a level 10 restriction. Tower shield is learned at level 16 so the requirement for the tc is lowered to level 10 for easier access.
