Panty filling and panty pooping sound the same, and stores list them separately for a reason.
Panty pooping
The moment itself: going in panties while dressed. The focus is on the act and the reaction.
Panty filling
The result: full panties, often held, shown and played with afterwards. These clips usually run longer and overlap with smearing.
Both have their own rows on the front page. Related: how messy scenes are made.