A series of marriage, a series of children, and how the status changes in her relationship to kohanim change her permissions, as it were, for terumah. Also, introducing the 10th chapter, and the cases that leave the relevant parties in limbo. But the mishnah presents this as pretty serious, even if the people didn't worry too much about it. Plus, the details that fray the marital bond, while the information they provide....which of course leads to halitzah in place of yibum. Note that "presumed dead" meant something different back in the day.