Dumb question: What is difference between "Applies to" subtab on custom field VERSUS "Apply to forms" (and selecting the particular form you want the field to show up on)? Obviously, you can apply to a form but have Applies To subtab have nothing checked. Vice versa, you can check e.g. Sales Order and then have nothing under "Apply to Forms"

applies to at the bottom determines which transaction type the field applies to. If the transaction Purchase is not selected, you wouldn't be able to Apply to Forms, of any Purchase Order Form.