"Notion of Use", "Fable", and "Power Level" are explained in Prometheum Exxet. A fan translation is kinda-sorta in the works. Basically, characters need to know how to use an artifact for its power to be used. The value used to determine how proficient a character is with an artifact is "Notion of Use". Notion of Use slowly increases as they use the artifact and it unlocks abilities for the artifact. "Fable" is used when an artifact is famous; if a character has an idea of what something is capable of, they are better at learning how to use it. Fable is a new secondary skill that is pretty similar to History. "Power Level" is a measurement of how powerful an artifact is. Here's everything I have on PE: it is not complete, and it is not 100% accurate. It is, however, serviceable.