For example, a game starts with two players at 3 stocks each. Player 1 KOs player 2, so now P1 is leading in stock. P2 then KOs P1, but not before P1 deals 31% damage to P2. While the stocks are evened up at 2 each, P2 is "losing" by 31% (a score that could be written as 2-2 0%-31%). P1 has earned 31% of slag.
Similarly, if P1 and P2 have the same number of stock and P1 KO's P2, but P2 comes back and KO's P1 without taking damage, it is said that P1 did not get any slag (on that stock).