Alpha is multi-modal, which means it switches between different modes depending on which type of document you are editing. Everybody who has used Emacs knows about this concept. The idea with different modes is to change Alpha's behavior depending on which kind of text you're editing.
Alpha is very customizable.
Much of Alpha's functionality is implemented in 'packages' or 'extensions'. Many of the functions bound to keystrokes, as well as many of the functions in the menus, are written in Tcl. Of course, you don't have to learn Tcl to use Alpha. You can have lots of use of Alpha without knowing anything about Tcl. But if you want to become a real power user, you should consider trying it out.