Each user in a domain has an account with a security identifier (SID) that uniquely identifies the user. When you rename an account, the SID remains the same. When you delete an account, the SID is permanently removed from the accounts database. If you recreate an account with the same name, it receives a new SID.
Windows uses the syntax domainname\username to refer to a user. The domainname can either be a domain or the name of a local computer. No two users in the same domain can have the same name, but the same user name can be used in different domains. For example, the names REDMOND\kim and HYDERABAD\kim refer to different user accounts.
Interix uses a "principal domain." When working with Interix utilities, if no domain name is specified, a user or group is assumed to belong to the principal domain.
The principal domain is a system-wide concept. By default, it is the Windows domain to which the computer belongs, unless the computer belongs to a workgroup, in which case the principal domain is the name of the computer itself. If the domain of a user or group matches the computer's principal domain, Interix omits the domain name when displaying the user or group name. Similarly, when a user or group name without a domain name is passed to an Interix utility or function, the Interix subsystem assumes the user or group belongs to the principal domain. If the user or group does not belong to the principal domain, then Interix displays the form domainname+username or domainname+groupname. When passing a user or group name with its associated domain, you can use either of the following:
The form domainname+username interferes less with Win32 utilities. You do not need to use any escape characters. The plus sign (+) is also not used in any Interix or Win32 path names. The forms domainname/username and domainname:username, which were supported in previous versions of Interix, are no longer supported.
If you specify only a plus sign (+) before username, the Interix subsystem treats the user as belonging to a special built-in account such as System, Everyone, and Administrators.
To view your principal domain, at the Interix shell prompt, type:
echo $USERDOMAIN