when you register a menu id, what you are doing is assigning an id to any menu which contains the string in your register_menuid command, ie-
Code:
register_menuid("\yThis is just an example")
when you use register_menucmd, all this does is assigns a command (argument 3) to any menu with the id in argument 1 when certain key(s) are pressed (argument 2).
so to answer your question, it's not the first set of code that determines the command to run, it's actually determined when the menu is 'registered' on plugin_init. (or wherever else the menus are registered)
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