On Tuesday, 21 June 2016, Diario Libre published a photographic display of what the lettering means on vehicle license plates. Basically, the E and Z mean Exonerated from taxes, of which there are many categories. A is for private automobile, and G for SUV, DD is for a vehicle still owned by the vehicle dealer, L is a cargo vehicle, C is a tourist taxi, P is a touristic bus, B is for interurban public transport vehicle, M is for funeral vehicles, T is for inter-urban buses, H is for ambulances, I is a private bus, U is for heavy machinery, W is for diplomatic vehicles, OI is for international organizations, OF is for Air Force vehicle, OP is for police vehicle, among others.
http://www.diariolibre.com/noticias/ciudad/el-significado-oculto-de-la-placa-de-su-vehiculo-YF4034076
