This is a floor prep layer that is nicely smoothed in order to lay and mortar the mosaic floor (SU 8380) on top of it. In a patch slightly visible near the threshold between rooms I-18 and I-17, it looks as if a patch of this floor prep SU may be covering an earlier stone block threshold (not yet recorded in Season 2018 because there are other stratigraphic layers on top of it and there is no more time to excavate it this season). This suggests that the floor prep (SU 8381) and mosaic (8381) may be part of a later phase of the building.