Since all of our material is governed by the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License (https://creativecommons.org/licenses/by-nc/4.0/), adding traditionally copyrighted materials in your Flexbook® textbook is not permitted without the express permission of that material's copyright holder. Content is governed by section 6.6 of our Terms of Use and shared publicly under the CC BY-NC 4.0 license. (Note that content that is licensed as CC BY, but *not* as CC BY-NC 4.0 cannot be used on our site.)
It is your responsibility to ensure content you add to a FlexBook® textbook is available under a license compatible with CC BY-NC, or is your original work. All CK-12 content is licensed in similar ways to allow for sharing and derivations.