Hi,
If you create different Addresses at the Business Partner level, then on the Contract Account in change mode (Tcode CAA2) :
You would have to select the preferred address from the drop-down list of Addresses in field JDC Addr. field (Technical field name ADDSC).
Once you select a an address that is different from the Standard Business Partner Address, the "Fix" checkbox is turned on.
I hope this helps.
Regards,
Ivor