copied from the midi spec doc

NOTE-OFF
MIDI provides two roughly equivalent means of turning off a note (voice). A note may be turned off
either by sending a Note-Off message for the same note number and channel, or by sending a Note-On message for that note and channel with a velocity value of zero. The advantage to using “Note-On at zero velocity” is that it can avoid sending additional status bytes when Running Status is employed.

Due to this efficiency, sending Note-On messages with velocity values of zero is the most commonly used method. However, some keyboard instruments implement release velocity where a Note-Off code (8nH) accompanied by a “velocity off” byte is used. A receiver must be capable of recognizing either method of turning off a note, and should treat them identically.

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