difference between inner and outer rear wheel wells?

OK, maybe I am just and idiot here.
I want to do a mini tub on my dart. I came across a post where a guy used replacement well housing and just welded them together, versus fabbing.

I found a site that sells wheel wells,
http://www.mattsclassicbowties.com/contents/en-us/d1884.html

But I am confused on the difference between inner and outer.
I am looking at the dart now, and I see a rear well, its under the car...so to me that means inner.
Aide from that it is the fender.

Am I missing something?