Quantizing percentage means how much closer to the quantization grid's perfect timing position the timing (position and/or length) of notes is moved.
- 0% : the timing is not changed at all. It's a no-change operation.
- 10% : the timing is moved 1/10 the way from its current position towards the exact quantization grid value closest to the current position.
- 50% : the timing is moved to halfway between where it is before quantization and the exact quantization grid value closest to the current position.
- 100% : the timing is moved all the way to the closest quantization grid value
Try it: quantize by 10%. Quantize again. Quantize again. Etc. You'll notice that the notes are nudged closer to the grid every time.
However, you might encounter the word quantization percentage (or Amount as it's called in Reason) in two different kinds of functions: destructive and non-destructive quantization.
- Destructive quantization changes the stored data in the MIDI (or audio) tracks. It's a "Quantize this now" operation that the user performs, and which can usually be undone with an "undo" operation immediately after performing the operation. Usually this means that when you perform such a destructive operation, you see the notes move in the MIDI (or audio) editor.
- Non-destructive quantization changes timing transparently, "on the fly", all the time as the sequence is playing. You can change the quantization amount to 100% and the notes will play back as if they had been quantized 100%, but they will still remain in their original timing locations in the stored MIDI data. You can change the amount to 0%, and the notes will start playing back in their un-quantized original way. Then to 50% ... and back to 0% etc. The note data will retain their original timing all the time. You probably won't see the notes move anywhere in the MIDI editor as you change the non-destructive quantize amount. (though it's possible that a sequencer could show the timing like it would sound when played back)
In Reason and Ableton Live there's a feature called Groove, which works as non-destructive quantization. In Ableton you can do an "Apply Groove" operation, which destructively bakes the groove into the note data.