MIDI
MIDI records which note, when it starts, how long it holds, and how hard it was played. The instrument on the track turns that into sound.
MIDI records instructions for an instrument: which pitch, when it starts, how long it holds, and how hard it was played (velocity). The synth or drum machine on the track turns that list into audio.
Change the instrument and the same notes play the new sound. An audio region already contains the recording, so a different instrument leaves it as it was.
A MIDI region on the timeline holds the notes for one passage. Open it in the MIDI editor to draw notes, quantize them, or edit velocity. Up to five regions can be open together. Drum machines with a step sequencer, such as Beatbox 9, also store patterns on the device. Those are note instructions kept in the machine.
A USB keyboard sends the same instructions. Record MIDI writes them into a region on the selected track. The on-screen piano plays the selected instrument without writing anything, so you can hear a preset before you record.