armrest is not continuous to the chair back, but is missing in the elbow area.
A couch, bench, or other arrangement of seats next to each other may have arm rest at the sides and/or arm rests in between. The latter may be provided for comfort, but also for privacy e.g. in public transport and other public places, and to prevent lying on the bench. Arm rests reduce both desired and undesired proximity. A loveseat in particular, has no arm rest in between.