Originally there were just Calendar events. Vetted in advance (w.r.t. controls and distance) by AUK, much like now.
Then someone created Permenant events that could be ridden at any time gathering PoP as required. Same vetting in advance (w.r.t. controls and distance).
Then someone created DIY Permanent events that were vetted upon application to the DIY organiser but still validated using traditional PoP.
Then someone created DIYxGPS events that were vetted upon application to the DIY organiser but validated by analysing the resulting GPX files.
So everything is either a Calendar event or a Permanent, with DIYs, DIYxGPS and ECEs all in the "Perm" basket.
(Not entirely sure where Arrows and Darts fit in, they may be in their own little classification, again due to history.)
Anyway, in what way is a "DIY" a subset of "perm"? Ipso facto a 'DIY' is NOT a 'perm'.
A DIY is actually short for "DIY Permanent".
A DIY is a perm because it is not a calendar event.