abstract is the default directory that is created using VisualSVN when setting up the repository, this is a long-standing convention for the structure of the repository:
(disclaimer: I'm not a svn lawyer)
trunk : will contain your main development line, here you will create new functions.
branches : these are side components for your development; branches are usually created from a trunk or other parent branch. Typically, you use branches to develop new features before merging them into a trunk or to perform maintenance tasks.
tags : this is just a place to fix the release for reference, you cannot (by default) overwrite branches created in tags
the main thing: in svn all abstracts are a directory, you can branch any existing branch to any other place (tags are limited), and you can merge them later (with more or less pain) into another branch
abstracts are all general descriptions, and there is a different scheme for using branches, if you need more information, thanks for being more accurate.
edit: I also recommend spending time reading chapter 4 Branching and Merging
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