Programming

When the enum reaches your JSON, it will have to be a string (as JSON does not have a dedicated "enum" type). But it at least ensures that languages parsing your JSON will should have a consistent set of strings to read.

Consider this small bit of Elm code (which you may not be an Elm dev, and thats okay, but it's the concept that you should look to get):

-- A Directions "enum" type with four options:
-- North, East, South, West
type Directions
    = North
    | East
    | South
    | West

-- How to turn each Directions into a String
-- Which can then be encoded in JSON
directionsToString : Directions -> String
directionsToString direction =
    case direction of
        North -> "north"
        East  -> "east"
        South -> "south"
        West  -> "west"

-- "Maybe Directions" since not all strings can be parsed as a Directions.
-- The return will be "Just <something>" or "Nothing"
directionsFromString : String -> Maybe Directions
directionsFromString dirString =
    case dirString of
        "north" -> Just North
        "east"  -> Just East
        "south" -> Just South
        "west"  -> Just West
        _       -> Nothing

The two functions (directionsFromString and directionsToString) are ready to be used as part of JSON handling, to read a String from a key and turn it into a Directions enum member, or to turn a Directions to a String and insert the string to a key's value

But all that aside, for your restructuring, and keeping with the license plate example, both type and license number could be contained in a small object. For example:

{
    ...
    "licensePlate": {
        "type": "government"    <- an enum in the language parsing this
                                   but a string in JSON
        "plateNumber": "ABC123"
        ...
    }
    ...
}