This section is from the "Version Control with Subversion" book, by Ben Collins-Sussman, Brian W. Fitzpatrick and C. Michael Pilato. Also available from Amazon: Version Control with Subversion.
Subversion allows users to invent arbitrarily-named
versioned properties on files and directories, as well as
unversioned properties on revisions. The only restriction is on
properties whose names begin with svn:
(those
are reserved for Subversion's own use). While these properties
may be set by users to control Subversion's behavior, users may
not invent new svn:
properties.