Hi yourus, just like you can have several images attachments inside one slot, you can have several attachments inside the same skin, but each needs to have their own skin placeholder.
A skin placeholder can only have one attachment per skin, but it can have several attachments spread out different skins.
Skins - Spine User Guide

In practical terms this means that if you plan to have different angles of the same arm, or head, you should keep them in the same slot so that you can alternate them when needed. If in addiction you have skins, each attachment will have its skins placeholder.
This means that if you have a complete set of heads for a skin, but only a head for another skin, when in an animation you switch images, a skin will show all the heads, while the other will show nothing for the skin placeholders you leave empty.

tl;dr: make sure you have the same set of arms for each skin, place the images in a slot, put each image under a skin placeholder, make sure the skin placeholders have an image for each skin.

Yes, the underlined one is a skin placeholder! what goes inside a skin placeholder will only be visible when the selected skin is active!