A quick search didn’t reveal anything about it on the forum. It appears to be just a text field which the user can populate. It may also be a destination for the “description” attribute for layers in imported AutoCad files.
Like many tools, users may find uses beyond what the developer anticipated. As far as I can determine you can put any text you desire in the field. That could be a description of the field, the date when it was added, in a document which multiple users work on it could be who added it, etc. Or it could be something with limited or no connection with Rhino.