So, you don't know what he means, either.
Regular pvc is very rigid and darn near impossible to use for a
sign, besides being extremely heavy. Expanded pvc has strength and can be used outdioors. It's bendable, easily cut with just about anything, drilled, glued and most woodworking tools make it all very easy. The link johnny provided explains all that to anyone who isn't sure.
When I said that I never heard that term, I meant in connection with ordering
sign materials. Any good distributor will know this and explain the options for the OP. However, if the vendor doesn't know, they'll most likely just sell ya something like DL mentioned.