I've seen quite a few people thinking Act 1 takes place after SB, Which considering the game's called 'Aftermath' it's fair to assume so.

Act 1 Takes place X years Before Security Breach
Act 2 Takes place X weeks After Security Breach's 'Good Vanny Ending'
Act 3 Takes place X weeks After Act 2

The game also takes place in a alternate universe to SB, but the way i write the story is combining Most Tales from the PizzaPlex Books, and all the games Before Security Breach, (Not counting RUIN, So the Mimic isn't a thing in Aftermath's PizzaPlex, Neither is 'Burntrap') And only the lore from the Good Vanny Ending.

I do like adding in things from the books, Nightmare gas, Illusion Disks, Etc are all fun to play around with in my opinion!

The reason i'm not adding in the mimic or Burntrap, is AFTERMATH was written before the mimic was a thing, And i just wasn't really a fan of Burntrap, as i felt Afton was a bit overused.

Hope this clears things up a bit! if y'all have any questions, let me know.