That's what Type Variables are explicitly for. They present the necessary semantics for bounded parametric polymorphism. In fact that's really the only reason they exist at all. The rest of the sub-type system works without them, but parametric polymorphism wouldn't work without them.

I don't know if the Erlang/Dialyzer docs cover that specifically, but I know the originial paper on Dialyzer and Dialyzer type specs does.