One inning is an opener, maybe two but that's pushing it, three or four innings is a short starter. The next guy that comes in is the piggyback assuming they're in to pitch multiple innings. You could also see short starter pitch like 3.1, stopper get out of the jam, and then the piggyback pitcher come in a clean inning. Of course all of this only applies if it's planned. A starter leaving early due to injury or ineffectiveness and a long reliever coming in to eat innings isn't really a piggyback.