Difference between Sapphire RX 470 4GB Ref and Sapphire RX 470 Nitro+ OC

Hello,

what are the difference betweeb this 2 sapphire RX470 4GB GPU in term of overclock/underclock and bios mod:

-- Sapphire Radeon RX 470 4GB Ref
-- Sapphire Radeon RX 470 Nitro+ OC 4GB

Thanks