According to The Book of Postfix: State-Of-The-Art Message Transport
by Ralf Hildebrandt, Patrick Koetter
"X-header is a generic term for an extension header field with a name that starts with a capith X and a hyphen. X-headers are meant to be nonstandard and to provide information only, and converstly, any nonstandard informative header should be an X-header. Here are a few sample X-headers (there are, of course, millions more):
X-Mailser: Xinian Evolution 1.4.3
X-spam-checker-version: SpamAssassin 2.53
Not sure if that exactly explains what DH is using it for, but it's at least a start. I found that from google book search.