ca31.pdf
Defines MIDI Continuous Controller 88 (58H) as High Resolution Velocity Prefix to the subsequent Note On / Note Off message.

High Resolution Velocity Prefix is intended to improve Note On/Off Velocity resolution while keeping compatibility with older instruments. High Resolution Velocity Prefix is intended to be used when 7-bit Note On / Note Off velocity resolution is not enough. In conjunction with this message, 14-bit resolution can be achieved.