Is the Signature parameter (e.g., in MtlX Multiply) documented anywhere? I was baffled by some of the choices. I did figure out what Vector3 (FA) meant.

The signature parameter switches the types of the node's inputs and outputs. It's a Houdini way of dealing with the same operation that has several variants, each differing only in the types it takes.

A MaterialX shader node can have several definitions that differ only in type it takes, and therefore such definitions fit well with Houdini's concept of a signature.

The signature name in Houdini comes form MaterialX name of the given definition (eg, "ND_multiply_vector2" and "ND_multiply_vector3FA"). We try to keep close to the original name, while making it a little bit more human-readable.

Thanks! Once I started looking around in the MaterialX library everything became clear.