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.
Formally, a Rigid Body is one that doesn't deform under stress. (i.e. It has a Young's Modulus of infinity).
Practically, it means that it's a body that doesn't bend or flex under normal loading.
"Rigid" just means the opposite of "flexible" and its meaning is subjective and context dependent. It's really more of a relative term: "drywall is more rigid than house wrap", "CMU walls are more rigid than drywall".