A CLE# is a legacy file naming standard we would use internally to keep track of your artwork as it was uploaded to our site to use for customization.
The CLE# would serialize that piece of artwork so we (and you) could find it easier when you returned to the site to reorder--i.e. you could easily use the same artwork you had already uploaded previously.
We have a very similar system in place on our new site. However, to separate the two so there was no confusion internally, we now use "AWXXXXXX" (you may have seen this if you've uploaded artwork and/or placed an order with us).
The CLE# is a legacy artwork tracking system that was only used on our previous site, however, for those of you that had placed recent orders on our old site and had uploaded artwork, we moved as much of those art files over as we could so you wouldn't have to type in a CLE#. However, we couldn't move everything, but still have all the old files internally on a server. If you have an old CLE#, you can use it when placing your order.