What is the meaning of "What are you supposed to be?"?

meaning

The mother was asking that girl that what her costume for Halloween is.

What are you supposed to be? = What are you dressed up as?

Related

Origin of "over-the-road trucking"
Common name for words like duplicate, triplicate, quadruplicate etc
Is there an expression or word for emotional/spiritual arrival?
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Replace "prevent" with something weaker
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Do I need a comma after "given that" and "in absence of"?
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Photosynthesizing (American English vs British English) [closed]
I will call somebody else, not neither him nor you
If I were or If I was [duplicate]
Idiom about choosing things