What are private and system identifiers?

I was learning c/c++ when i came across private and system identifiers.I have no idea what it means any help? I know what identifiers are but what are those.
i know that private start with an underscore and system start with double underscore.
I haven't heard of private starting with an underscore. If you are talking about private for a class/struct then it just means that only that class/struct and it's friends can access it where as protected the derived can access also and public anyone can access.
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