Board Thread:Wiki/@comment-27441457-20160627051627/@comment-5774380-20160627205927

There is no clear line, these situations need to be approached case by case.

In Ms. Kirstein's case, she first appeared in the extra pages of Vol. 6, and she referred to Jean as "Jean-Boy"; or at least with the word that in fan translations was spelled as such (in the official Kodansha USA version she just says "Jean"). She is a canon character that was later depicted in a non-canon source.

The same situation applies for the Iron Bamboo and Iceburst Stone. We have no way to know if any of those items was a creation by the Before the Fall staff or Isayama's creations that they adapted into their work; and even if it was the first case, the fact that Isayama took two Before the Fall items to the canon storyline doesn't necessarily mean that he took the whole story with them. Those two elements can perfectly exist without anything else.

No Regrets is a totally different situation. We got the first hints that the story was canon in the afterword that Hikaru Suruga included in No Regrets Vol. 2, where she thanks Isayama for "letting her write Levi's spin-off"; suggesting that Isayama had accepted it as Levi's official background. Then in chapter 76 we got a direct flashback of a No Regrets scene, and then in chapter 80 Furlan and Isabel crossed over on the main storyline. Both the flashback and the characters' presence would have absolutely no meaning if the events of No Regrets never happened. That's why this manga is now accepted as canon.