I think the phrases 'mid-arm' and 'long-arm' are pretty much interchangeable these days. There are no 'rules' just like there are no 'quilt police'.
All longarms use leaders of some sort, and there are many different ways you can attach your quilts to them. You have to do some research and find the best methods of everything for you - leaders, attaching quilts, quilting, etc. There are too many to get all answers in one place...but you should try the majority of them to find the one that works best for you.
Debbie in Austin