Contrary to the definitions in the other answers, I'd assume that this refers to selecting positions from the database where that's the only difference in mater

Contrary to the definitions in the other answers, I'd assume that this refers to selecting positions from the database where that's the only difference in material - so "clean up a pawn" means that all the other figures are the same, that there isn't also a bishop vs knight difference, for example. I really doubt if there was any attempt to check if there's some "strategic compensation" - how would you do that at a large scale? I doubt that even running a solid chess engine evaluation on all these positions is feasible, you need something where you can simply/cheaply filter positions from the database and then just count the winrate.