Choice of Roman numerals to indicate sleeping quarters (4)

I believe the answer is:

dorm

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'sleeping quarters' is the definition.
(dormitory)

I don't understand the remainder of the clue.

(Other definitions for dorm that I've seen before include "Rest area" , "Communal sleeping place (colloq.)" , "crash site" , "Boarding school bedroom (abbr)" , "Sleeping quarters (abbr)" .)

I've seen this clue in The Times.

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