From the back of the book:
Now you can explore the origins of the most powerful, but hardest to control, type of magic: Arcane. With this companion, the old rules for Arcane magic are completely revised to be 100% compatible with the RMSS. While some of the concepts from previous companions were maintained, the entire structure of Arcane magic has been revised.
In Arcane Companion you get:
A discussion of the concepts and premises behind Arcane magic and how GMs can introduce it into their games.
Guidelines on how to mesh the rules for the Arcane into the RMSS, how to handle temporary enchantments, and the risks of using Arcane magic.
Four new Arcane-using professions.
Rules for controlling the acquisition of Arcane magic.
46 Arcane spell lists.
Four new spell attack tables and a revised Basic Spell Attack Table.