What Type of Soil Can I Use in the Flats?
Use the brand you prefer. Some bags will say "for seed sowing" or something similar on them. Stay away from bags of soil that say weed-free as they may contain chemicals to prevent germination of weed seeds in that soil. Those chemicals will also prevent the germination of sown seeds.
You may reuse potting soil or seed sowing mix after it has been sterilized. Put the sowing medium into a flat and dowse it thoroughly with boiling water. Allow to cool to room temperature before sowing your seeds.