I.e., "generated" is a property of an instance of a field, not of a field; that's why the code sample you showed has a separate call to proto_item_set_generated().
That may not be the case for *all* fields that have generated instances, but there might be some fields that have both generated and non-generated instances.
For an example, take a look at packet-tftp.c
TFTP packets with the read request and write request opcodes contain a source or destination filename, respectively, and in those packets the corresponding fields are added without set_generated()
For packets with other opcodes, conversation tracking is used to add the source or destination filename, as appropriate, for the ongoing transfer, and there set_generated() is used because it's not actually in those packets.
John Thacker
