I don't understand why some of the examples shown her have a minus sign in front of the field name.

Because in a shuffle-by-year you want LOW numbers (years) first; in a shuffle-by-most-often-played you want HIGH numbers (play counts) first.

Why don't you simply allow negative factors? This would result in:

shuffle0=Most recently played,TIME=-16,RANDOM=1

or similar lines instead of the (at least for me) less readable

shuffle0=Most recently played,-TIME=16,RANDOM=1

cu,
sven