The gemara gives a number explanations for Rabbi Elazar’s opinion about why a Cuti can’t marry a Cutit. How far back does one need to check the lineage of the woman’s family he is planning to marry? It depends on if she is from the family of Kohanim or not. Why does one need to check her family but she doesn’t need to check the man’s family? There are all sorts of people whose family lineage doesn’t need checking. The gemara explains each one.