Which field of the IPv6 header allows for a dual-stack host to decide which stack to process the packet in?

The first 4 bits of an IPv6 header contain the version number. In an IPv4 packet, this is set to 0100, but in an IPv6 packet, this number is set to 0110. This allows for the host to decide which stack to process the packet in. Flow labels in IPv6 explain to a router how to handle packets and route them; they do not contain a version number. The source and destination addresses in IPv6 packets will be IPv6 addresses, but this is not how a dual-stack machine decides which protocol to use.