For MIDI regions, there are two types of quantization: Classic Quantize and Smart Quantize.
Classic Quantize quantizes MIDI notes to the nearest beat, using the current Quantize setting and any advanced Quantize parameters.
Smart Quantize works similarly to Flex Time for audio regions. MIDI events near grid positions (defined by the Q-Range setting) are analyzed. Based on a combined weighting of proxity to the target grid position and velocity, a reference point (”center of gravity”) is set for each group, and moved to the target grid positions. All (nearby) MIDI events move proportionally to the reference point. How far each event moves is determined by the Q-Strength setting. Unlike Classic Quantize, all MIDI events keep their original order, similar to an audio region edited using Flex Time. This includes not only notes but all MIDI data types (such as Pitch Bend and Control Change events).
In many cases, Smart Quantize can produce more natural sounding results than Classic quantization. For example, in a MIDI piano performance, the notes of quickly arpeggiated chords keep their relative positions, as do sustain pedal (on/off) events. Another useful situation is in MIDI drum performances that need some quantizing, but where classic quantization would distort rolls, flams, and other notes not played strictly on the beat (and also possibly the relative position of Hi-Hat on/off events). In both cases, Smart Quantize keeps the relative position of note on, note off, and CC events.
In the Region inspector, click the Quantize label, then choose a quantization type from the pop-up menu.