PrimaryGroupInherited defines whethere the group sid is inherited (true) or not (false) 

Indicates whether the primary group information was derived from a default mechanism. If this value is TRUE, it is default information, and the function stores this value as the SE_GROUP_DEFAULTED flag in the SECURITY_DESCRIPTOR_CONTROL structure. If this parameter is zero, the SE_GROUP_DEFAULTED flag is cleared. (source: http://msdn2.microsoft.com/en-us/library/aa379584.aspx); 

The Control flag is only updated in a newly created SD allocated by Create_SD.

TJwSecurityDescriptor Properties