It limits tags to the most used ones, compared to the most used tag. For example you have the following tags assigned:
- rock: 30 times
- folk rock: 25 times
- alternative: 4 times
The most used tag is rock, which was assigned 30 times. This is our maximum. The other tags get compared to this. So we get the following percentage values for the tags:
- rock: 100%
- folk rock: 83% (25 / 30)
- alternative: 13% (4 / 30)
If you set the percentage to 70% Picard will use both rock and folk rock. Setting it to 100% usually means you will only get the most used tags (it could still be multiple, though).
I just checked the default values: The last.fm plugin sets this to 15%, which gives a preference of using more tags even if they seem to be off. Picard’s own tag support has a similar option which defaults to 90%. This prevents outliers from being included in tags. I think a higher value is actually a better default, unless your goal is to have as many genres listed as possible 