The sentence says “may”. And the previous sentence says “ A housing unit is vacant if no one is living in it at the time of the interview, unless its occupants are only temporarily absent.”
I guess we would have to read further on how they determine temporary. But given the sampling method, I would guess people doing a door to door survey and noting it as temporary absent if it’s well kept?
> In addition, a vacant unit may be one which is entirely occupied by persons who have a usual residence elsewhere.