I just registered to necromance this thread, because it keeps coming up in Google searches about detachments. It is stated many times by several posters that detachments don't count towards core, which is not true. Since many people still play 8th edition, there may be others like me who could be mislead by this false information. I'm still new to Warhammer, and I almost believed what was said in this thread. To protect others who are in a similar situation as me, I therefore commit the sin of necromancing.

Page 30 of the English language army book for the Empire (2011 edition) reads:
"The points cost of a Detachment, and any upgrades it has taken, counts towards the points total of the same unit category as the Regimental Unit they are purchased with."Just to make sure I didn't misunderstand the wording, I also checked the German version of the army book (also printed in 2011). On page 30, it reads:
"Die Punktekosten einer Abteilung und jedweder ihrer Aufrüstungen
zählen zur Gesamtpunktezahl der Einheitenkategorie, zu der auch ihr dazugehöriges Hauptregiment zählt."
Since German is my first language, I can vouch that the German text is in perfect agreement with the English text.
The French version also agrees:
"La valeur en points d'un Détachement et de ses options
compte dans le total de points de la catégorie du Régiment auquel il appartient."
Moderators, you can ban me or lock the thread, but please, please, please don't delete this message, because this information really needs to be out there, for the sake of those who still play 8th edition!