Perhaps: "A vertical-bar-separated list of select header field names and copies of header field values that identify the header fields present when the message was signed. It is not required to include all header field names and values."
I've added essentially this wording. Sorry for the confusion; it was definitely ambiguous.
eric _______________________________________________NOTE WELL: This list operates according to http://mipassoc.org/dkim/ietf-list-rules.html