An abstraction hierarchy can treat a lower level as a black box, provided it upholds guarantees that the upper level can rely on.

“Effectively in an abstraction hierarchy to basically n to basically make a statement that the lower level just has some guarantees. I don't fix what the lower level even does like I don't care. It has to uphold a certain thing and that is something it's a constraint that I can basically rely upon when I build my abstraction on top.”