User blog comment:B214/Volume 22 spoilers/@comment-73.160.58.215-20160723140310/@comment-9595671-20160724044412

So basically the Extra Demons are any Demon names that aren't associated with the 72 pillars or connected with the Demons who make the Satans in DxD.

That makes sense, I guess if the 72 pillars theme was important from around the series' get go then everything else would be extra. Thanks for the help.