A more thorough explanation:

A Patent and Trademark Depository Library (PTDL) is a library that has been officially designated by the U.S. Patent and Trademark Office to provide information and resources to help people conduct patent and trademark searches.

For example, if someone wants to apply for a patent or trademark, they can visit a PTDL to access books, databases, and other materials that can help them research and prepare their application. These libraries are staffed by knowledgeable professionals who can assist with the search process and answer questions about patents and trademarks.

PTDLs are important resources for inventors, entrepreneurs, and businesses who need to protect their intellectual property. By providing access to valuable information and expertise, these libraries help to promote innovation and economic growth.