Overview#SID string is a LDAPSyntaxes with an OID of 126.96.36.199
SID string is found in the schema for the regarding attribute. This OID is described in the Microsoft Active Directory Technical Specification MSADTS. SID string corresponds with the 188.8.131.52.4.1.14184.108.40.206.40 syntax.
SID string is, as far as Ldapwiki can determine, exclusively used in Microsoft Active Directory.