idk - I just use whatever I have handy. Parchment paper usually works for me. My favorite is pattern paper. I just trace the design onto it & can do a very large section of the quilt all at once. I've also cut apart 2-gallon Ziplocks on 3 sides and used that in a pinch. It doesn't lay perfectly flat, but it's pretty close & very easy to see through or move. Sometimes it gets a bit static-y; if so, just grab a dryer sheet & rub it across the surface one time (do NOT do this if you don't plan to wash your quilt later as sometimes it can transfer a tiny spot of fabric softener to your fabric).
Parchment paper takes heat fine (it can even go in the oven). Pattern paper will take a little heat -- enough to get fusible to start sticking to fabric, but you'll still want to press a bit longer once you remove it. Ziplock bags obviously can't take any heat at all, so you only want to use those if you are doing applique glue or wash-away double sided tape or such.