Some EMAC devices do not support offloading of multiple protocol's CS's at once for TX. Only a single protocol's CS can be computed for any given packet.

Since, for example, in a UDP/IP packet, there is both a UDP CS and and IP CS that needs to be done, and each must be placed in the respective protocol's CS field in its header, you have to choose one.

In order to support this, new fields are needed that must be filled in by the stack, which give the h/w the information it needs to compute the CS's for TX packets.