Example

A little example of this is, the Linux Kernel. (although it is held in CVS)
There are thousands of people working on it in branches, but only one person is capable, of adding new features into it's trunk. Tags are created between releases of new Kernels (example: the mm-sources). And releases are always named with "`2.6.4,2.5.5"' and so on.
So you see, Linus Torvards is the Lord of the trunk, and he is the only one who can make a release. All the others can only work in branches, and can only make tags.
Pretty cool, ey?