So "Chorus Line" has a line on the stage; "Hello, Dolly!" a staircase. They are inherent in the material as written, required by the form.
How is the staircase "inherent in the material...REQUIRED by the form?" The staircase is iconic/traditional, and therefore expected by the audience. It does get two very "blink and you miss them" mentions in the dialogue. And of course Dolly's big entrance for the title song is incredibly effective with that staircase. But I'm not sure I'd go as far as "inherent" or "required by the form."
I can be sure that a clever director could have the Harmonia Gardens be at street level, with another effective way to stage Dolly's entrance that could work just as well - even amid clutched pearl murmurs in the audience of "what, no STAIRCASE??!!??!!" ;-) I'm not recommending that, just that it really could be done if carefully thought out.
I'm just saying that there really is a viable difference between "inherent/required" and "iconic." And yes, sometimes iconic is indeed the right choice to go with - but not always the only one.