[hyperthesis]

Thanks for your guidance! I now see they're (now-obviously!) things like TCP and http. I had missed their informal definition:

> Here we use the term protocol to denote a specification of the interaction patterns between different system components. [...] To give a more concrete intuition, an informal specification of a protocol for an e-banking system may be as follows: The banking server repeatedly offers a menu with three options: (1) request a banking statement, which is sent back by the server, (2) request a payment, after which the client will send payment data, or (3) terminate the session.

I would say it's like how you use an API.