[otrs] What is CustomerValid / validID?

Hi, everyone. I'm new to OTRS and the list. I'm setting up an OTRS installation that will use an MS SQL backend for CustomerUser. So far, so good. However, I'm mystified about the CustomerValid / validID / valid_id field. I'm clear on the other mappings (unique, customer name, etc), but I have no idea what to map this valid thing to, if anything. I have googled and RTFM'd, but can't find any reference to the thing. Please advise! -b