Well I need to give support the benefit of the doubt sometimes. I put a ticket in (after everyone’s responses here), and they answered it quite well this morning.
There’s an article, KB0030729.
But short answer is, it’s not used and obsolete, but can be used in a customization if desired.
I’m lost as to how they mean a BAQ, but I’m not adventurous with finances anyway.
Text of KB article:
The General Ledger GL Control assigned through the Modules tab within the Site Configuration for Inventory Management is not used in out of the box rules and is not loaded as part of the VBD for COS and WIP Capture. Even if you have some GL Controls assigned there it is not used in the current rules and you will not be able to include it in the input document unless you were to use a BAQ. It is obsolete for our Standard Posting Rules, but technically you can create a customization that can use it (though not in really convenient way - BAQ).