Well, USB can I guess be split in layers too, as can the TCP/IP thingy. And googling on 'USB stack' shows that it isn't an Amiga specific term. In fact, the term USB stack probably isn't used much as in the major OSs it's seamlessly into the OS itself. If you dig back a few year in Windows you'll find that the earlier W95 (spit) versions didn't include an 'USB stack'.
Here's a link to a third-party NT4 USB stack.