Autonumbering allows the ability to have several prefixes, but how can the correct one be selected w

I am creating customer contacts and would like to use different auto numbering prefixes depending on whether the contact is for a grants customer(GRT) or for a subcontractor (SUB).  Under Set Up Financials > Common Definitions > Codes & Auto Numbering > Auto Numbering, I selected the CONTACT_ID field.  On this page, I added 2 codes GRT and SUB.  The check box for Auto Default automatically checked the 1st row - GRT.  So when I added a new Contact under Customers, GRT populates as the mask.  If I manually check SUB, then SUB appends itself to the contact ID.  The CONTACT_ID field