Note: please forgive the crude diagrams and terse descriptions in this help file. It is only a first draft.
The left chooser contains a list of categories for a particular archive indexed by the browser. Generally speaking, each category represents an independently developed project.
Clicking on a category name presents you with a list of branches within that category. Generally speaking, one branch represents "the official sources" (conventionally named the "devo" branch) while other branches represent more advanced development that will eventially be merged into the official sources.
If you click on a particular branch, the branch list will move to the left chooser, and a list of specific versions within that branch will appear in the right window.