Can someone explain as detailed as possible what, exactly, a rigid surface is? The context in which I am inquiring is in regards to Green Glue. It requires application between two rigid surfaces. I am sure this is a global term that doesn't apply to just Green Glue.
Rigid means something that will hold its shape on its own. Wood (except veneer), drywall, backer-board are all rigid.
A piece of thin paneling made from plastic/fiberglass would not probably count.
However, the definition is somewhat subjective. In the context of Green Glue, I presume they want to ensure no movement between the layers.