CTO UI is an application architecture framework that CrypTool uses to create and maintain its plugins on CrypTool Online. CTO UI makes it easy to create interactive plugins by providing the foundation for plugin creation. CTO UI takes care of the data flow, rendering and updating of the user interface. In fact, with CTO UI you should be able to create an entire plugin with a single file.
CTO UI is built on the same principles as React, but optimized for CTO. If you are familiar with React, then you should be familiar with CTO UI.