My rectum and anus were completely removed. My disease was Crohn's Colitis and confined to my colon & rectum, with no small intestine involvement. I didn't want to leave anything in there that could potentially lead to a recurrence since my Crohn's was pretty aggressive.
The way I understand it, the rectum and anus are both removed through a lengthwise incision made from enough past the girlie part to wherever to get the rectum and anus out. My surgeon said my incision was stitched up on the inside for part of the way, except for the last 1/2 inch or so along the length of the incision which was left open to heal from the inside out. It has been 2 years and I have never looked, so I don't know what it looks like, but it "felt" like a flap of skin.
My perception and use of the term "Barbie Butt" is just that it is closed with no opening and not the perfect-ness of Barbie's butt. That would have been nice, though