別べっ室しつ

〈人ひと〉を別べっ室しつに案あん内ないするto show someone to a separate room
スタッフはお客きゃく様さまを別べっ室しつに案あん内ないした。
The staff showed the customer to a separate room.
別べっ室しつで〈事こと〉を行おこなうto hold (a meeting, interview, etc.) in a separate room
面めん接せつは別べっ室しつで行おこなわれた。
The interview was held in a separate room.
別べっ室しつに呼よばれるto be called into a separate room
彼かれは突とつ然ぜん別べっ室しつに呼よばれた。
He was suddenly called into a separate room.
別べっ室しつで待まつto wait in a separate room
患かん者じゃは別べっ室しつで診しん察さつを待まっていた。
The patient was waiting for the examination in a separate room.

Kanji

側そく室しつ sense 3
側室 is the standard modern word for a noble's concubine, while 別室 in this sense is an archaic, rarely used synonym.
個こ室しつ sense 1
個室 emphasizes a private room for one person's exclusive use, while 別室 simply means a room apart from the main one, regardless of privacy.
控ひかえ室しつN1sense 2
控室 specifically means a waiting or green room, whereas 別室 more broadly refers to any room set aside for a special purpose.
隣りん室しつ
別室 means a separate room used for a different purpose, whereas 隣室 specifically means the room physically adjacent to the current one.

同どう室しつ
別室 means a separate room, the direct opposite of 同室's meaning of the same room.

私し室しつ sense 1
私室 refers to a person's own private room, while 別室 refers to a separate room used for a specific occasion, not personal ownership.

Compound of 別 (separate) and 室 (room). The archaic concubine sense derives from the idea of a separate living quarters.