From your list it is first and mostly:
1. a plaintext file format (.urtext) - specifically, a syntax
2. a Python library to parse & manipulate said file format (syntax)
These in turn require some implementation within an editor. Sublime Text was chosen for its built-in Python interpreter, its package install system, and its GUI features that together comprise a low barrier to entry.
Thanks for the feedback, we will try to make this clearer.