What's true core difference between DHCPNack VS. DHCPDecline?

If DHCPNack is (Server-to-Client)

And

DHCPDecline (Client-to-Server)

But they both have similar purposes, that if it finds another devices is using the same IPV4 or it's a duplicate IPV4.

Are there any true core difference besides that the fact that both are either client to server or server to client?

Thanks