The SID property contains the security identifier (SID) for this account. a SID is a string value of variable length used ...

The SID property contains the security identifier (SID) for this account. a SID is a string value of variable length used to identify a trustee. Each account has a unique SID issued by an authority (such as a Windows domain), stored in a security database. When a user logs on, the system retrieves the user's SID from the database and places it in the user's access token. The system uses the SID in the user's access token to identify the user in all subsequent interactions with Windows security. When a SID has been used as the unique identifier for a user or group, it cannot be used again to identify another user or group.