CUI
Short answer
The AutoCAD command that opens the Customize User Interface editor, where you edit menus, ribbon, and shortcuts.
CUI (also CUILOAD/CUIUNLOAD) is the AutoCAD command set for managing customization. Typing CUI opens the Customize User Interface editor — a tree view of all loaded CUIX files where you create new commands, drag them onto ribbon panels, assign icons, and set keyboard shortcuts. CUILOAD and CUIUNLOAD load and unload partial CUIX files at runtime.