The degree of contamination. Samurai only are contaminated some, but the eta were those who were severely contaminated by constantly doing work/living in conditions that required contact with things that squicked out the sense of cleanness of the 16th and 17th centuries. That would basically deal with things having a lot to do with death and blood. But for some reason some other odd jobs got associated with uncleanness, like making bamboo tea whisks, and being certain entertainers....
It’s a weird combo of Shinto/Buddhist prejudices codified by the Shogunate, and possibly used as a scapegoat class, to give the peasant farmer, who was pretty low on the totem poll himself, something to be above....and it also worked that you could be put into the non-person class as a punishment for a long time...
Yup, them bamboo tea whisks are pretty disgusting.
I understand that potential marriage partners are still screened in Japan for eta ancestry, although the government tries to pretend it doesn’t happen.