It contains essential data used for routing telephone calls through the Public Switched Telephone Network (PSTN). 

• Routing Information: The LERG provides detailed information on the network interconnections of local exchange carriers (LECs), including routing codes and network endpoints. This information is used to facilitate the efficient routing of telephone calls.

• NPA-NXX Codes: The guide includes NPA (Numbering Plan Area, commonly known as area code) and NXX (central office code) data. These codes are critical in determining the path that a call takes from its origin to its destination.

• Use by Carriers: Telephone companies and other telecommunications service providers use the LERG to program their switches and routing equipment, ensuring that calls are correctly and efficiently routed across various networks.

• Updates and Maintenance: The LERG is regularly updated to reflect changes in network configurations, new entries, or modifications due to the expansion of services and infrastructure by LECs.

The Local Exchange Routing Guide is a fundamental resource in the telecommunications industry, providing the necessary data to ensure proper routing of calls and the interconnection of diverse telecommunication networks.