Mobile Radio District is exactly what has been described: old etymology from past days when common frequencies were used by law agencies for interagency hailing, communications, etc. Some system owners have carried over these names when they migrated off analog VHF/UHF and did their initial fleet mapping of their trunked radio systems. For example, the old KIC334 was Marietta Police's old VHF radio system and hasn't been in wide use since 1996. The licenses are maintained as is some minimal infrastructure, and MPD explorers use it from time to time, but you won't find a VHF radio in any Marietta Police car these days- only 800. They had a talk group in their fleetmap on 17D labeled "MPD MRD 800" but it rarely got used if ever.