From the Foreword:
The book you are about to read could be described as "the fourth book of the trilogy". Whereas the three Revised Mazes & Minotaurs rulebooks previously published by Legendary Games Studio (the Players Manual, the Maze Masters Guide and the Creature Compendium), gave you all the rules needed to play countless perilous expeditions and heroic quests, the Mazes & Minotaurs Companion is a collection of optional rules on a wide variety of topics.
'Optional' is the key word here : each Maze Master is free to decide which of these rules they want to use in their M&M games; like all other books and modules published by Legendary Games Studio, the Companion is a fully official M&M product, but 'fully official' does not mean 'mandatory'.
In keeping with the spirit of the Revised edition, most of these optional rules are streamlined or (sometimes heavily) reworked versions of rules that first appeared in original, pre-Revision supplements (like the venerable Men & Monsters and Myth & Magic or the more recent Unveiled Addenda) or in the columns of Griffin but this book also includes 100% new, never-before published material.