User blog comment:B214/Question and Answers 6/@comment-125.209.182.182-20170124142434/@comment-3435913-20170124163421

I think it's closer to an irregular since it accumilates the power of Fafnir according the the description. If it was a subspecies, then the sacred gear itself would have exhibited a notable difference to its regular counter-part, like Siegfriend's Twice Critical resembles a literal dragon's arm as opposed to the vanbrace form we presume is similar to Issei's Boosted Gear 1st base form we see in the anime.