From the introduction:
Different role-playing rule systems can be applied and/or used with the New Horizon world and its different games.
Volt is a role-playing system developed to be used for different genres and worlds, with only a few adjustments for the setting. Volt has been adapted for New Horizon and is the official rule system for this book. This book is using version 3.0 of the Volt rule system.
Volt is a set of rules and practices designed to quickly and accurately determine the realistic outcome of a character's action, taking both skill and circumstance into account. The abilities of a character are summed up on the character sheet, and chance is represented by two twenty-sided dice (d20). No other dice or tables are used in-game.