I believe that, technically, they are custom tags. They just happen to be a bunch of extremely useful ones that ought to have been made 'standard', but aren't. So you can prefix them with anything you like, but if you decide to use anything other than 'c', your co-workers will be silently (or not-so-silently) cursing you.
Cheers,<br />Roger<br />SCJP 1.2, 1.4, SCBCD 1.3, SCWCD 1.4