The securityIdentifier attribute contains a String(Octet) representation of the SID belonging to the trusted domain. This value contains the domain relative SID ([MS-DTYP] section 2.4.2) of identities issued by the trusted domain. For example, for "example.com", a trusted domain, the value may be S-1-5-2223345-6677. The domain administrator for example.com would have a SID of S-1-5-2223345-6677-512.
This attribute is unique on all TDOs within the domain. The system rejects attempts to create a duplicate value.