Foreign key to subtype or super types? I'm using SQL Server

and in this case i have the following tables:


Restock: Foreign key to an R&amp;D Staff member
R&amp;D_Staff: subtype from Staff
Sales_Staff: subtype from Staff
Shipping_Staff:  subtype from Staff
Staff: supertype


Should the foreign key be pointing to Staff (Supertype) or to R&amp;D_Staff (Subtype)

Thanks in advance
Re-phrasing, do you mean:


  don't allow Sales or Shipping staff values in restock?


Then the foreign key is from Restock to R&amp;D_Staff to allow only values from RnD staff in the Restock table