Bought a Rein BKN0016R CV boot kit for a MK4 Golf.
However, it came with a C-shaped metal clip(?) that I have no clue where it goes or what it does.
Didn't see a similar piece on the existing boot or axle.
Middle piece, between clamp and Belleville washer.
What is it ?
Is it only needed for car models other than mine ?
I don't know for sure, but it might be. If you look at the models which can use this part, there's a ton of them. It was used on a wide variety of VW/Audi lines from about 2000-2020. If you don't see the clip as you pulled your old one apart and it isn't in the diagram, I would not worry about it. Manufacturers often include a small part and even duplicate parts within the same kit so it can be sold and work on different models. It's cheaper/easier for them to create a more universal kit than to create specific unique ones.
Oh, I was wrong.
It appears to be an equivalent to part 17 in the inset exploded diagram.
Didn't make the connection at first, because it looks quite different in shape and style to the diagram's "circlip", but this C-shaped open ring is a retaining clip / securing ring.
It keeps the axle shaft from escaping the outer joint.