SSN Formula: 

US Customers:
=If Not(IsNull([SSN])) Then Left([SSN];3)+"-"+Substr([SSN];4;2)+"-"+Right([SSN];4)

CA Customers:
=If Not(IsNull([SIN])) Then Left([SIN];3)+"-"+Substr([SIN];4;3)+"-"+Right([SIN];3)

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