From what I understand of subversion if you have a repo that contains multiple projects, then you can branch individual projects within that repo (see SVN Red book - Using Branches)
However what I don't quite follow is what happens when you create a branch in one of the distributed systems (Git, Hg, Bazaar - I don't think it matters which one). Can you branch just a sub-directory of the repo, or when you create the branch are you branching the entire repo?
This question is part of a larger one that I posted on superuser (choice and setup of version control) and has come about as I am trying to figure out how to best version control a large hierarchal layout of independent projects.
It may be that for distributed systems that what I would like to do is best handled by a sub-project mechanism of some sort - but again that is something I am not clear on although I have heard the term mentioned in regards to git.
With git, a branch is simply a pointer to the commit at the tip of the branch. It doesn't contain any information of its own. So, your history might look like this:
- o - o - o - o - o (branchA)
\
o - o (branchB)
Each o there is a commit, which represents the state of the entire repository at that point. The two branches thus in general represent different states of the entire repo, though it could be that they only differ in the contents of one subdirectory. There certainly won't be any wasted space, though; if two commits use the same version of a given file, they internally point to the same object for its contents.
Depending on what you're actually trying to do, you could be interested in using submodules, which are essentially a mechanism for placing repos inside of repos, so that you can have a meta-project repository which contains sub-project (embedded) repositories.