SU 8394 is a layer of signinum used to seal the surrounding walls off for the creation of a basin in the northern half of Room I-22 (with a bottom formed by SU 8393 and, later, SU 9395). It belongs to a relatively late phase of the building, and is similar to other basin features (e.g. SU 8408 in Room I-17), all of which appear to be related to a new industrial or productive function.

The signinum lines the walls and extends below the top siguinum floor (su8393), indicating that the room was intended to hold liquid. It is uncertain how much liquid could be held in the room, because it is uncertain how tall the walls extended.