For the SID associated with a user account, that last number (1000) is the RID (Relative ID). Everything before it is the SID of the computer or the domain.
So SID(account) = SID(computer/domain) + RID.
If an RID is smaller than 1000, then you know it is associated with a default security principal, like the default administrative account (500). Non-default accounts have a RID of 1000 or larger. Typically RIDs are assigned sequentially: 1000, 1001, 1002, ...
With this method, Microsoft tries to guarantee that SIDs are unique.
The SID for a computer or a domain (like S-1-5-21-3623811015-3361044348-30300820) is generated with an algorithm designed so that it's virtually impossible that the same SID would be generated by 2 different computers.
By appending an RID that is generated sequentially and never reused, SIDs for other security principals (SID(computer/domain) + RID) are also unique.
There can be cases where the SIDs are not unique, e.g. duplicated.
One case is where disk imaging is used to install several computers that are not domain members: first on one computer, Windows is installed. Then the disk of this computer is cloned onto the disks of other computers as a method to rapidly deploy Windows on a bunch of computers. If no actions are taken to change the SID of the clone (with utilities like sysprep, newsid, ...), then all clones will have the same computer SID, and thus will have user accounts with the same SID.
Since the SID ends with 1000, it's the first non-default SID created, and I think is unlikely that this is a domain account, and more likely to be a local account. Unless it's a domain with very few users.
SANS ISC Senior Handler
Microsoft MVP 2011-2016 Consumer Security, Windows Insider MVP 2016-2019
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