This filter only exists as a Sell Filter.
It will sell Positions that no longer rank in the top N%, based on the Ranking Rules.
So, let’s first discuss about instruments’ rank: all instruments that have passed the Buy Filters will be ranked according to the Ranking Rules, and the top X instruments will be entered as Positions (according to the parameter “Total Positions to Hold”).
These top X instruments belong in the top N% of the ranked instruments (which means not all top N% instruments are entered as Positions). So, if during the course of trading a Position's rank drops below the top N%, then this filter will exit that Position as there are more profitable instruments in its place.
The only parameter for this filter defines the threshold top-percent for the ranked instruments.
Beware, if you set this too low, e.g. 1%, the “Total Positions to Hold” may never be fully filled as the majority of the instruments will hit this Sell Filter (at the same time as they pass the Buy Filters), thus they're excluded from being bought. That's because Portfolio Boss is smart enough to exclude instruments that hit the Sell Filters even before their Positions are entered.