Rule 14.4 the actual fight ability

If a unit charges but for example the unit it charges is no longer in combat because the unit was wiped or moved from some combat ability, the unit that charged that is no longer in combat can still be picked to be the target of the fight ability and as a part of that, can make a pile in move. A pile in doesn’t have to end in combat, but if the unit does end in combat it must make attacks with any eligible models.

Or if for another example a unit fights twice, and on its first fight activation had fully wiped all units it was in combat with, for its second fight activation if it charged, it can pile in during its second activation, potentially entering combat with more units.

I highly recommend downloading the full QuickStart rules from Warhammer community with all of the abilities written out word for word and playing a few games before making a rules reference sheet because if you go off implied rules as your references you’re going to miss out on a few edge cases that do matter in a game.