A CLAP instrument is a CLAP Plugin that can convert MIDI events to audio. You can select an CLAP instrument by clicking the instrument slot's down arrow. The CLAP instrument plugins appear in the list's Plugins section.
MIDI 2.0 / MPE
CLAP instruments can use MIDI 2.0 protocol messages, CLAP note expressions or MPE. CLAP note expressions support per-note Pitch Bend, Volume, Pan, Expression, Brightness, Vibrato Depth and Aftertouch.
It's up to the plugin to tell the host which protocols it supports and which one it prefers to use.