“Discrete” mode in MIDI automation tracks also plays another role.
MIDI automation consists of a series of discrete, individual MIDI Continuous Controller (“CC”) messages. These can be recorded from incoming MIDI data, or entered graphically. In the latter case, it is typically desirable to have Ardour interpolate between the points and send some extra messages so that the change from one value to another is smooth.
But this is not always desirable, and so you can put the track into “discrete” mode where the only outbound messages during playback will be the ones actually present in the data. This works well when you have recorded MIDI CC data from a controller of some sort, or when you actually want “step-wise” changes in a particular parameter of whatever receives the CC data.