On my zen+ mode0 is the processors highest power/frequency mode. It can be used to overclock without breaking the auto. up/down clocking feature.
Sadly it doesn't seem to work on my b350 based motherboard with my 3700x. Maybe it works on x570 based boards? I'm crossing my fingers AsRock will fix it in future BIOS upgrades, but don't really expect anything on my dated budget board...