These playmats would show up in the cosmetics tab, and you would associate a playmat with your deck by similar means to associating a card sleeve or coin to your deck. Designs may include a particular design for specific Pokemon types, a design for Energy types, a design for each starter, simple decorative patterns, or other designs. When you play TCG with someone else, your side of the board would have this pattern over it. Your cards would show up on top of this pattern.
This means that when you are playing TCG with someone else, your cards don't just have to sit on the ground, over water, or hovering in the air. Also, it would provide another thing that people can customize with datapacks.