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So I've seen and continue to hear that players are using the specialist detachments from the Vigilus campaign books in Tournaments and in Matched Play.

I think these are fantastic Narrative play campaign books, but I do not see anywhere in them that says the specialist detachments (or datasheets for that matter) can be used for Matched Play and I'm struggling to understand how and why it was just assumed by so many (or am I actually missing something) that Narrative based rules and detachments would automatically be OK to use in Tournaments.

Note that GW's 2019 GT event packet does not allow the Vigilus books to be used; at all. "Publications in use: All current and in-print Warhammer 40,000 Index books, Codexes, beta rules, FAQs and errata from Games Workshop and Forge World..."

Check the update listed on the community site https://warhammerworld.games-workshop.com/the-official-2018-2019-warhammer-40000-grand-tournament/

it specifically says Specialist Detachments may be used.

Yup. That'll do it. Thank you.