I think this is trying to be a discriminated union type[1] in json. One way to do it is to have a ‘tag’ field that says “ok” or “err” or whatever and then other fields for the rest of the data (or an array with the tag in the first slot). Another is to have one field whose name is the tag and whose value is everything else (which is what happened here).

[1] eg one where the possible values are either Err(<some error>) or Ok(<some result>), and the data inside could be more complex types instead of just strings