Was experimenting a bit more and it looks like Kevin was right too with respect to sub selection more generally in the form playing a role. Depending upon if I select the US sub or ones in any of Europe/Hong Kong/Japan the Taxable box will pop up in the form (defaulted unchecked) only for the US-based sub. I don't think Nexus plays a role tho because also I tried a Japan sub with a Japan country in the address and it didn't make the box appear (both in the form and after save). So, the taxable field seems to be basically US-specific. This feels like it ultimately comes down to what I assume was iterative development of Advanced Taxes coming out after they had a structure in place.