• I agree that Resource is a system database with the physical name mssqlsystemresource.mdf, in the sense that the system uses it. But I can also see a physical file named distmdl.mdf that is not mentioned in the msdn article, but apparently is used as a model for the distribution database if you set up replication, so isn't that also a system database?

    OTOH if you open SSMS you can only see the four databases that most of us think of when you say 'system database'.

    I think the msdn article needs some clarification.