Menus.
A menu is a chest-like window with rows of slots, each showing an item with its own words. A click is an event with the slot and the item, never a trade of the item: the rule decides what the click does, such as giving the item for a price.
Commands.
A command is something players type after a slash, with typed arguments: text, a number, a player. The rule on the command reads each argument as a field. A name the game already uses is refused.
Who may.
A command is for everyone or for operators, and its arguments are checked before the rule runs: too few, too many or the wrong kind answer with the usage line.