Not sure if this is exactly what you are looking to accomplish, Bruce, but in the attached V7 BSV:
1) Create some regions using "New region" menu option
2) Create customers using "New customer" .. you will see "Region pick" switch
when Region Pick is turned ON, you will be prompted to pick a region
3) Edit customers - when editing, you will also be promoted to pick a region only when Region Pick is turned ON.