Subaru Oil Separator Gasket

Found an interest part at the Subaru EJ20 engine rocker cover area, I think not all the EJ20 series have this part but what they call it as rocker cover oil separator, for the above diagram, check it out on 11831 and 11832.

And for this rocker cover show above, can you see the black cover on the upper left of the rocker cover?

The first diagram which show item 11831 is the above item which is the black cover you can find beside the rocker cover, and the below photo show the item 11832 which is the gasket sit between the above black cover and the rocker cover, and they call this as oil separator gasket.