The short explanation
Telnet sends plain text over a TCP connection. No encryption, no encoding, nothing clever — which is exactly why it has survived in AV control, where the commands are short and the network is your own.
You connect to an address and a port, send a line of text, and the device answers with a line of text.
Where you run into it
Control systems speak Telnet. A processor from Crestron, Control4 or a home-automation script can hold a Telnet session open and drive a device with the same commands you could type by hand. That is what makes an AV device automatable rather than merely remote-controllable.
It is also the fastest way to test an idea. Before writing a macro, you can send the command manually and see exactly what comes back.
How the DR HDMI 8K handles it
The DR HDMI 8K accepts IP and Telnet commands on port 2201 by default, and HTTP commands as well. Commands are lowercase and end with a carriage return or newline; every response comes back with both. The complete command list ships as a text file inside every firmware package, matched to that revision.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.