I googled it. According to
this thread, the "system image" is the img file suitable for use with fastboot. In contrast the other partitions listed (including "system") have to be used with twrp. So I think system and system image are the same thing but you would use different tools to restore them back onto your phone. I'd think most people would save time on their backup by skipping "system image".... that can often be obtained elsewhere if you need it (and you don't need it if you can restore system from recovery).