Incomplete SO and Customer Master

Hi,

I have a scenario in which , in the sales order, field tax code (customized field under Header level of SO in Tab Additional Data A)gets picked from bill to party Customer Master (Tax Classification). Whenever the Tax Classification field of Bill to Party in Customer Master is blank , blank value would flow to SO and hence SO will be treated as incomplete . How can I make this Tax Classification field of Customer master mandatory ? So that value 0 /1 is maintained , such that the TAX field in SO would not be left blank .

Provided some screenshots .

Thanks in advance