Get to know the worlds
Many knowledge skills are going to be somewhat fluid as they can change from shadow to shadow. It should be known that knowledge in a certain area not only provides a character with said knowledge in regards to their home shadow or personal experience but also grants them the ability to experiment and quickly learn how their area of expertise changes from shadow to shadow.
This skill has a couple of uses. First, it determines what a character knows about a particular shadow or world that can be found out in Shadow. From large, powerful, or important countries and kingdoms to knowing the layout or common legends about the shadow. For example, this skill might be able to tell someone certain things about Shadow Earth, like some of the most powerful countries (at the time of this writing) are The United States, China, Russia, and Europe. The largest continent is Africa. The apex residents are human, it's currently in the information age era, it has little to no magic, and is likely closer to Amber in Shadow than The Courts of Chaos. Even more information could be gathered from this skill but I think you get the idea.
The second use is knowing how to handle shadow stuff, which is basically the essence of the multiverse. One with the proper powers is able to harness shadow stuff and manipulate it so they can create items or even creatures. This skill will determine what a character knows about utilizing shadow stuff and can be used to create advantages for the Craft skill when crafting items or creatures from shadow stuff.
Keep in mind this skill is important for magic users as it will provide them with information about their ability to use magic, how strong magic is in a given shadow, and even if they are able to bend the rules, such as if a shadow doesn't naturally have magic but outsiders could still use it.