Can be placed under a 10" x 20" seedling tray for seed germination. It will constantly stay at 21 degrees celsius, and shut off automatically when its reached the heat point. 

Seeds require the heat for proper germination, without it your success rate will be lower. 

10" x 20" heat mat can fit 1 tray.

20" x 20" heat mat can fit up to 2 trays

20" x 48" heat mat can fit up to 4 trays.

Note: Once seedlings are starting to form their true leaves remove the heat mat to ensure you don't overheat the roots.