Branches are separate entities with their own courses, theme, users and domain.
Each branch has its own URL. For example, if you own a domain mydomain.training.atmeyer.com and create a branch b1, then its own URL is b1-mydomain.training.atmeyer.com.
Only members of this branch can login from that specific URL. Users that self-register through that branch become its members directly.
The user that initially created a domain is by default a SuperAdmin (full privileges) on branches as well. A regular branch admin has a limited number of admin tools on branches (namely access to Users, Courses and Reports).