> When I plug in an HDMI cable, I know it's going to be used for display output
https://www.cmple.com/learn/learning-center-hdmi-cables-with...
> Firewire? A client peripheral connected to a master computer bus
Nope. Firewire is masterless/multi-master/peer-to-peer. That's why you could not only transfer music from you laptop to an iPod, but from one iPod to another iPod without any (other) computers on the bus. It is also why two computers can by attached via Firewire, as a generic network link or for low-level debugging.