Hi Tim,
Skin placeholders act as placeholders for what will be shown. Their contents varies depending on which skin is active.
To get the result you want, you just need one skin placeholder:
Activate the red skin, and place the red jacket under that skin placeholder.
Next, activate the blue skin, you'll notice the skin placeholder is now empty. Place the blue jacket under that same skin placeholder.
If you need to have a version of the jacket that belongs to that same skin but is a different pose, for example a jacket as seen from the front and one from the side, then you'll need a skin placeholder for both versions of the jacket. Both will be filled by red jackets from the front and from the side, and the same goes for the blue ones.
I hope this clarifies a bit better how skin placeholders work. If you're in doubt I think giving a peek to the user guide videos, or the mix and match project may help too!
Skins - Spine User Guide
Skins view - Spine User Guide
Mix and Match example
