An appraisal sheet looks like a mere blank sheet of paper. They are extremely valuable, so much so that even noblemen can't quite get their hands on them. Only someone with the status of the Holy Knight like Lugh or the hero like Epona can have it thanks to their authority, but for others it's very difficult.
No physical laws, magical theories, or scientific mechanisms can explain how it works. This phenomenon is a mystery, or a miracle, so to speak. There exist only a very limited number of people who can make those appraisal sheets, and they are heavily protected. It's even rumored that they aren't made by human hands. Indeed, for Lugh, there is no way humans can create something like that, and supposing a human can make them, they would have to borrow the power of something supernatural for that.
To use them, you have to clutch your appraisal sheet and focus on it. Doing so, the names and descriptions of your skills appear. This is what has happened with Lugh, Dia and Tarte so far. Another aspect is the fact that the appraisal sheet doesn't show the skills you gained through 【My Loyal Knights】but only those you were born with.