Advanced Transformed Subversion
Atsvn is not a fully-valid SVN client. It is a shell with a graphical interface above the original svn console client.
The main purpose - to allow work with individual files of different revisions from different branches and even svn repository. Atsvn's working copy consists of one or more separate files without reference to the folder structure. Even the names of files in receiving a copy may differ from the original. The files are under version control - you can update them, commit your changes, the same as svn may.
To provide flexibility to work with files atsvn uses a local repository, a staging area. From the standpoint of the svn this is a working copy. For a user local repository is completely transparent.