I think what it means is that you can use a double needle with it if you want. I have never sewed with a double needle, except in class.
Also, I have a regular size machine and have been able to do quilting on it. It is tough but many of us have "normal" size machines. I have a long-arm on the wish list for "someday"
Many times the foot is already there, just called an "embroidery" foot or something like that. You want to be able to drop or cover the feed dogs and a Walking foot is a foot that you will want to acquire.