create

What the editor's Add menu creates. edui.create makes an empty entity, each built-in primitive the mesh registry resolves (cube, sphere, plane, ...), a point light, a spot light and a camera, five metres along the editor camera's view and no lower than half a metre above the ground, as one undoable step, and selects what it made.

declare puts each of them in the editor command registry under the add menu (add.cube, add.light.point, add.camera, add.empty, ...). The top bar's Add menu and the Hierarchy's + menu both list every command declared under add, so a world or a panel that declares one of its own (commands.declare{ id, title, category, menu = "add", order, run }) appears in both.