Should have been defined (along with more information about the data set), but I assume it means that the opponent has no tactic that wins them material. For i

Should have been defined (along with more information about the data set), but I assume it means that the opponent has no tactic that wins them material. For instance if I just take their protected bishop with my bishop then I'm briefly up a bishop. So it would not include that situation or where the trade is more complicated.