One of the essential conditions of comfortable work is the convenience and speed of accessing add-ons functional. It is not enough to write add-on and register it in Blender API system. Be sure to give the easily call add-on functions ability to user. So, any addon should determine a place to put interface buttons that provide access to its functionality.
All available Blender user interface located on panels. Three panels are the most used: T-bar – opens/hides by pressing the t keyboard button, N-bar – opens/hides by pressing the n button, and the Properties panel – called in the main menu of any window typed it as Properties.
Blender API allows to define users panels to place add-on functional buttons on any of this three main panels.