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. Default designs may include designs for specific Pokemon types, designs for Energy types, designs for each starter, designs for different Legendaries, simple decorative patterns, visually-appealing artwork, or other designs. When you play TCG with someone else, your side of the board would have this picture over it. Your cards would show up on top of this picture.
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.