The affordances of digital textbooks go beyond using textbooks in the four walls of a traditional classroom. With digital textbooks, users can swipe the text to scroll; increase/decrease size of text and images and change brightness options; view and read downloaded information; interact with 3D models and images; make notes, search topics, follow hyperlinks, take quizzes, self-regulate learning and "synthesize" own textbooks and library, etc. However, there are some constraints such as overload of working memory, depending on digital devices' functionality, etc. The correlation between affordances and constraints is analyzed through identification and description of eight didactic systems with platforms maintaining learner-centered environment, interactive feedback, social media, user interface design, and desired results. In this chapter, the concept of "affordances" as a way of strengthening eight possible models explored, called "didactic systems". The used methods are "thematic evaluation" and "comparative analysis". The conclusion and future research is provided at the end.