See Local Exchange Routing Guide.

The LERG contains information about the current network configuration and scheduled changes within the local exchange providers' networks. The LERG is primarily designed to be used for routing of interLATA calls by inter-exchange carriers. The LERG informs telecom companies which end office or tandem office the NNX resides in and how calls should be routed and rated so that they can properly terminate to the appropriate telephone number at the proper rate.

The Local Exchange Routing Guide. A database managed by Telcordia and used for all routing instructions. Assignments define which NXX is assigned to which switch and allows for rerouting of calls to a different switch from which it was originally assigned.