An abstract syntax tree copy expression represents the an assignment of one expression to another which causes / requires a copy operation. Within the RigVM AST this is only used for copying work state out of / into parameters or when composing / decomposing a structure (for ex: assigning a float to a vector.x). In C++ the copy expression is used for structures, for ex: FVector A = B; The copy expression references two pins / a link from the model.