This question is a spin-off from “Is you’re the door on the right. grammatically correct?” . After the original question, some ideas came to me, about its conditions and construction. I opened this question to see if my ideas are worth considering and to organize them.
"There." They had reached the second landing. "You're the door on the right. I'll call you when it's over!" And she hurried off downstairs again. (Harry Potter 5 [US Version]: p.62) [Bold font is mine]
N.B.: The speaker is Mrs. Weasely, who led Harry to his room.
1. Just from the sentence You’re the door on the right., would you have an impression there’re other people than you?
(I believe that the form of the sentence implies the existence of a group of people.)
2. In new situations (This time, the subject is I )
1) There’s no assignment.
The rooms are available on a first-come-first-served basis. Mrs. Weasley brings Harry upstairs just to let him know where the rooms are and to tell him he can choose whichever he likes. In this case, can Harry say like this? :“Oh, I see. Thanks. I’m the door on the right (meaning ‘I’ll use the room on the right side’)”
2) There’s an assignment.
The guests including Harry knew the layout of the rooms and already decided the assignment of the rooms by themselves. Mrs. Weasley, who doesn’t know which room is assigned to whom, brings Harry upstairs just to let him know where the rooms are. In this case, can Harry say like this? : “Oh, I see. Thanks. I’m the door on the right (meaning ‘my room is at the door on the right.’)”
(I believe that some assignment in advance is one of the most important conditions of this form, whether the subject is I or you. Therefore if such assignment doesn’t exist, the acceptability of this form, I think, is lower or zero. I would think 1) is less acceptable than 2).)
3. Can Mrs. Weasley in the top citation say “You’re the room on the right.” instead of “You’re the door on the right.”?
(I believe that the subject of this form includes the thing which assigned to him or her. I wouldn’t think she can say you’re the room on the right. because the subject you already has room in it.)
I know these questions are not enough to understand the form completely, but I hope I can get some clues about its conditions and construction. Thank you.