And here, so too with an ibur, this same way applies: It is impossible to enter into an Adam (to 'ibur' into him) and gilgul into him, [because] only three Nefashot can come to help the Nefesh that is [in] this Adam. So with him, there are four [Nefashot], but not more than this. However, it is possible that there are fewer [Nefashot] than this. It is also possible that they can come in the form of an actual gilgul ('sod gilgul mamesh,' perhaps referring to yibbum).
Each of them (the Nefashot coming in the form of ibur) come for their own needs, to rectify whatever minor sins they did (so they come in the form of a gilgul which has the 'citizenship status' of an ibur, and they are called an actual gilgul because they come at time of birth). Or, they come to complete whichever mitzvah they are missing (so they reincarnate as a complete gilgul).