The UserSID property indicates the SID of the user for whom this startup command will run. That User property may be empty ...

The UserSID property indicates the SID of the user for whom this startup command will run. That User property may be empty but UserSID still has a value if the user name can't be resolved (like in the case of a deleted user). The property is helpful to distinguish between commands associated w/ two different users with unresolved names. It may be NULL when the command is associated with items not actually identifying a user like All Users.
Example:S-1-5-21-1579938362-1064596589-3161144252-1006