In that security challenge 1 we are asked to choose 3 "account structures". I didn’t pay attention to the terminology as I hadn’t read yet the documentation pointed in the Tips references: AWS Multi-Account Security Strategy https://d1.awsstatic.com/aws-answers/AWS_Multi_Account_Security_Strategy.pdf
However, that document dates from 2 years ago and – although the concepts remains (like a centralized logging account, a centralized shared services account…) remains – many things have changed since then in AWS recommended multi-account strategy including also the "Account Structure" naming. Even in 2018 the recommended multi-account strategy was different than what is the provided PDF…
I think the re:invent 2019 video: "AWS re:Invent 2019:
"Architecting security & governance across your landing zone (SEC325-R2)" https://www.youtube.com/watch?v=zVJnenaD3U8
is a far better reference.
Some of these items are on our list to update, but do realize that just because AWS comes out with new services, questions on the exam don’t change overnight. It’s quite common for exam questions to show up on the exam out of the vast database of questions which cover a wide range of "generations" of recommended approaches. This is why we don’t immediately rewrite course content when new services or whitepapers are released.