No, they are materials. Trust me though, you don't want to get into adjusting orthotics, it's a major pain in the ass. Especially when you introduce solvents to ppt, poron, certain met pads etc (to take them apart to adjust) because the solvents breaks down the material and you end up having to replace the entire thing.
Among other materials; in terms of coverings we use plastazote, vinyl, spenco, and thermosky.
plastazote - normally use for diabetics/ people with sensitive feet, lacking fat pads.
vinyl - for the more active person, because it doesn't wear as easily and does not absorb sweat.
spenco - for your average person that is of moderate activity level (it's has a thin cloth-like covering to it and is a bit cushiony)
thermosky - I don't cover any fo's in thermosky because it falls apart easily, only time I'll use it is if we have a pt waiting and i have to make some adjustments to the orthotic I'll just cover it overtop instead of having to strip down the entire fo.
I'll normally use 1/8'' poron --> sulcus on all my FO's just to add a bit of cushioning to it -- beware tho, like i previously mentioned, if you have to go in and make any adjustments, when you use the thinner to pull back the cover it eats the poron right up.
Then of course there are different types of eva-based framings, we use according to pt foot anatomy, age, activity level, etc.
Let me know if I can be of furhter help.
cheers