Along with availability: :always and availability: :on_request, we could have availability: :optional pad.
A pad with availability: :optional could either exist or not exist, but there would be at most one instance of such a pad.
The pad could be referred from the element's implementation with just plain name, without any dynamic identifier, which would be a benefit over using availability: :on_request pads, where we need to be aware of the dynamic id.
Along with
availability: :alwaysandavailability: :on_request, we could haveavailability: :optionalpad. A pad withavailability: :optionalcould either exist or not exist, but there would be at most one instance of such a pad. The pad could be referred from the element's implementation with just plain name, without any dynamic identifier, which would be a benefit over usingavailability: :on_requestpads, where we need to be aware of the dynamic id.