Magento 2 Attribute Property – How ‘user_defined’ and ‘system’ attribute propety are different?

‘system’ attribute property tells if attribute is system defined or not.

‘user_defined’ attribute property tells if attribute is custom (define by developer) or not.

It means, if ‘system’=0, the attribute is ‘user_defined’ and vice versa.

So why Magento 2 uses two property (system, user_defined) for the same reason ? If they are different then how ?