Difference between "System" keychain and "iCloud" keychain in Keychain access for OSX

Can anyone explain what the 'System' keychain is in comparison to the 'iCloud' keychain on my Macbook?

When viewing the WIFI networks lists they appear to be almost identical within 'system' keychain and 'iCloud' keychain.

How do these relate to each other?

My assumption is that, with iCloud keychain enabled on both my iOS device and my macbook, is that any wifi network I connect to on my iOS device is added to the 'iCloud' keychain and then transferred also to the 'system' keychain. Is that right? If so, when does this happen?

When I have 'forgotten' a network on my iOS device, this seems to remove it from the iCloud keychain as expected, but will this network remain in the 'system' keychain viewable within keychain access on the Macbook?

Also, the 'date modified' data seems to be odd, with lots of networks listed as modified on the same date and time - what would cause this please?

It would be great if Apple could include a 'last accessed' column.

Many thanks for your help.