On Aptitude is an extension to the All Us Gamers Inner Core Rules.
After you have mastered the Dice Engine the next question from players will be, "how do I know what my character's aptitude is?"
This module gives a formal mechanism for giving characters attributes and skills, a system for developing skills over time, and some extended action methods for cooperating characters, sequences of actions, taking the hit for a friend that has failed a save, and taking time to automatically succeed at an action.
This does not alter the Inner Core, but extends it, and keeps to the generally easy to remember and adjudicate style of the Inner Core. As with the base rules it is not required that you develop a character before starting play. The character may emerge as play progresses.
(DriveThruRPG.com description, https://www.drivethrurpg.com/product/303914/All-Us-Gamers-On-Aptitude-Rules)