According to Lawrence Berkeley National Labs and the Department of Energy, a Milligrid is a type of microgrid that involves a segment of the legacy regulated utility grid. This differs from a true microgrid, which are wholly on one site with a clear geographical boundary. Milligrids involve a way to deploy distributed energy resources so they can be directed to critical infrastructure in the case of an emergency.
Baltimore Gas and Electric is testing a community microgrid in downtown Baltimore that will back up power to grocery stores, pharmacies, clinics, gas stations and public buildings that could serve as shelters in the event of an emergency. It is a milligrid because it involves multiple points on BGEs grid that are not necessarily in the same boundary. A milligrid does not need a single point of connection with the utility grid, as many microgrids have.