This is the charge for any extra person per room product and is directly linked to the default capacity of that room. If a room only allows 1 person, then there is no multi occupancy charge. In contrast if there are 6 people allowed in a room, then from the 2nd person onwards, a charge can apply subject to the property policy.
You can change the daily multi Occupancy charge per product and per day in the detailed product view under sales plan:
