On a normal MIDI channel, a modulation — vibrato, a filter sweep — moves every held note together, because the controller belongs to the channel, not the note. Per-note expression gives each sounding note its own controllers, so a chord can shimmer with every voice moving independently. That's the idea behind MPE (MIDI Polyphonic Expression), MIDI 2.0's per-note controllers, and CLAP's note expressions and polyphonic modulation. Play a chord below and flip between the two.
.mid — each note on its own channel with a pitch-bend to
its exact tuning, which is per-note tuning expression — and the
modular patchbay models modulation as control voltage. CLAP's note
expressions (see the plugin-formats page) are the native,
any-parameter evolution of the same idea.