Whats a rear main seal?

Your rear main seal is the last seal or first depends on how you look at it. but any way its the seal thats on the crank just behind the flywheel. if its not leaking there is no need to replace it.

Is are main seal a one peace or two peace?
I had to change one once and it was a one peace one, and I think I had to take the whole crank out, to change it.
It was a pain in the ass to do. I think it was on a caviler hatch back.
Kinda hard to remember. it was a long time ago almost five years ago.