Following information for your reference:
Members of this group are managed by the system.
Default User Rights: None
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I tried to find more information, but without luck.
I will do more research about it, if there are any latest information, i will update here!
And is there a reason why anyone would ever want to change a user account to "System Managed"?
The information about this group is really limited.
I only find the following information about the account: