I wrote it.
The idea is this:
1. blank-master is the template that drives some of the basic functionality, such as form field input focus, linking in CSS stylesheets, developer-support, translator-mode, etc., that master template authors would otherwise easily mess up, or they wouldn't keep their versions up-to-date. It doesn't show anything inside the browser frame, though (apart from translator mode/developer-support stuff), leaving completee design freedom to the template designer.
2. site-master gives you the top navigation, handling things like a link back to the index page, which isn't a link when you're on the index page already; login/logout/your workspace/admin links.
3. default-master goes all the way, to include context bar, page title, etc.
4. packages/acs-subsite/www/group-master is an alternative to default-master. It still uses site-master, but it adds a set of tabs for navigation inside the subsite.