What document contains non-regulatory material common to most ACS documents?

The ACS Companion Guide for Pilots is designed to provide supplementary information that aids in the understanding and application of the standards outlined in the Airman Certification Standards (ACS). This guide typically includes practical information and tips that help pilots prepare for their certification exams and understand the expectations set forth in the official ACS documents. It serves as a useful resource by offering context, clarifying concepts, and detailing best practices that aren't strictly regulatory but are essential for successful pilot training and certification.

The other documents mentioned, while valuable in their own right, do not serve this specific purpose. For instance, the Pilot's Operating Handbook is primarily focused on the operational limitations and necessary information for a specific aircraft, while the Airman Certification Manual provides official regulations and processes related to airman certification. The Pilot Certification Handbook encompasses broader rules and requirements in a general sense but lacks the non-regulatory, supportive elements that the ACS Companion Guide offers.