This is not a full-fledged compendium about MIDI as such. If you are looking for details about MIDI in general you may start at https://en.wikipedia.org/wiki/MIDI or at https://www.midi.org/. Here, we'll concentrate on Titan's capabilities to be controlled by MIDI, or to control other devices by MIDI.

Some basic trivia:

• MIDI was invented in the early 1980s, and is thus older than DMX
• originally it was just a way to control electronic musical intruments from other instruments/devices
• essentially MIDI is a signal connection. 'Real' MIDI is a 5pin cable, the connector being a round 'DIN' connector. But over the years, many developments have occured. The most important thing is USB-MIDI: transmitting MIDI information over USB interfaces. Then, there is a way to route MIDI over IP networks (rtp-MIDI), MIDI data can be stored in files (not relevant for Titan), and more.
• unlike DMX which strives to semi-permanently send the status of a whole system (i.e. a dmx universe), MIDI only transmits events: a key has been pressed or released, a parameter has changed etc. Hence, the data rate of MIDI is much smaller.
• MIDI messages usually comprise of up to 3 bytes. One bit of every byte is used to determine whether it's a status byte or a data byte. Hence, the 'resolution' or MIDI is 7 bit, or 0…127.
• the specification explains in great detail which message translates to which note or other value. This way you could be sure that when you pressed e.g. the note C on a MIDI keyboard this triggered the C on a sampler or expander.
• Nowadays there are also other uses for MIDI, e.g. MIDI Show Control (triggering cues in a show), MIDI Machine Control (sending transport commands to e.g. audio workstation programs), or MIDI Timecode

In general MIDI has been implemented in Titan consoles right from the beginning. However, there are some limitations. E.g. the Titan One and T1 do not provide for MIDI at all, the T2 can only do USB-MIDI (as there is no hardware MIDI outlet), and the Titan Mobile and Quartz can only receive MIDI but cannot send it (as there is no MIDI output provided). USB-MIDI was only added in Titan v12, output via USB-MIDI in Titan v13.

Here is an overview: