User interface extensibility allows you to create custom graphical user interfaces (GUI) and controls for STK to provide user-defined workflows which can combine STK and in-house functionality. You can extend the AGI products in a couple of different ways.
The simplest way to extend the STK user interface is to write HTML pages and load them from the internal HTML viewer in STK. If a tighter integration with the STK user interface is required, user interface (UI) plugins are also available. UI plugins allow you to create your own toolbars, context menus, and windows that look and behave identically to standard out-of-the-box STK user interface components.
STK 11.2.1 Programming Interface