File this one under the "I almost put a fist through my monitor" section. :h_a_w:  Just fought with this one for a couple of hours, but finally figured it out- and I wanted to post here so that it might help someone else avoid a mind numbing, rage inducing Tuesday morning

I recently created a new custom Record with a bunch of fields, one of which relates it  back to a Customer Record.  For a while, I was considering making that Customer a Parent of the custom Record, and I wanted it to appear on a Subtab.  So I created the custom Subtab and, created a List/Record field, checked the "Record is Parent" box and selected my custom Subtab under the "Parent Subtab" drop down.