It's an interesting question., and like the others have said, the answer can vary from teacher to teacher. I think close embrace is the farthest from the ballroom frame. A lot of the structure of the connection comes from the chest, and the arms are mostly for support. As far as open goes, the one thing I can tell you is that when I first learned AT with open, there was a large emphasis on dynamic/flexible arms. There is a structure, but it moves and shifts with the dancers. A lot of tango movements are circular or move in spirals, internally as well as externally. The leader's right arm slides around his partner, and the follower's left moves up and down his arm or around his body as needed depending on the movement. Open and close aren't really set terms... it's not a 1 or a 0, though a lot of people dance it that way. The embrace is a continuum. If you go into modern/nuevo, they take the flexible embrace a step farther... the concepts of compression, momentum, and balance are explored. Movements that you might find more appropriate for bolero or swing become possible.