Explanation of the "flow --json" output

I'd like to know what the output of the `flow --json` command means.
Like
- why there's a certain number of **"message"**s
- why do some of them have a line number equal to 0 (e.g. `{"descr": ..., "level": "error", "path": "", "line": 0, "endline": 0, "start": 1, "end": 0}`)
- what exactly does **"kind"** mean (I've only seen `"kind": "infer"` so far and do not know what other kinds there are)

Is there any place I could learn that?

`flow --json --help` does not produce any meaningful information.