That's right. You can of course make changes to 7BO and save it as a custom preset. If for example you want it to have a bigger butt you can tweak the appropriate sliders to your liking and then click Save As. A window will pop up and you can name the preset and tick the UNP-related check boxes. I usually tick all 4 of these boxes because I don't know what is the difference between them, except for the HDT part of course:
There's a lot of things you can do with UUNP including making combinations of bodies. Have fun with it and experiment. For example you can make a preset that is 50% 7BO and 50% UNPBO, or anything else. You can even make combinations of 3 or more body types as long as the total % of all the bodies adds up to 100, no more no less.