A more thorough explanation:

The Manual of Patent Examining Procedure (MPEP) is a book that contains the rules and guidelines for patent examiners at the U.S. Patent and Trademark Office. It includes both substantive law, which refers to legal precedents, and procedural rules, which outline the steps that examiners must follow when reviewing patent applications.

For example, the MPEP might provide guidance on how to determine whether an invention is eligible for a patent, or how to evaluate the novelty and non-obviousness of a proposed invention. It might also outline the steps that examiners must take when communicating with patent applicants, or how to handle disputes that arise during the patent examination process.

The MPEP is an important resource for patent examiners, as it helps to ensure that patent applications are reviewed consistently and fairly. By following the guidelines outlined in the MPEP, examiners can help to ensure that only valid and valuable patents are granted, while also protecting the rights of inventors and promoting innovation.