A 401k plan document is the official written set of rules that outlines how a company’s 401k retirement plan works. It includes details like who can join the plan, how contributions are made, when employees can take money out, and how the plan complies with government regulations. This document acts as a legal agreement between the employer and the government, and it must be followed exactly. If a company wants to change how their 401k plan works—like adding a Roth option or updating eligibility rules—they must update the plan document to reflect those changes. It’s the foundation that keeps the plan running fairly and legally.