Why have both RU7 and RU8?

[v8kid]

I'm probably being a bit dim and risk embarrassing myself but in the book drawings of an independent rear upright the part the sierra bearing carrier bolts to is a 10mm thick part numbered RU8.

On the upright there is another 10mm thick part numbered RU7 which is welded to the upright.

Why is RU8 needed? Its heavy and does not appear to add to the rigidity.

I see both parts have different internal diameters but on my Lobro driveshafts there are no steps or projections requiring this. Quite the reverse the additional 10m width interferes with the abs sensor location.

Am I missing the obvious here? I had one suggestion that RU8 was only a spacer to ensure alignment in which case can I replace it with ally?

Cheers

David