Interesting that they're using JSONPath, which isn't even specified formally anywhere. The only other major implementor that I know about is Kubernetes, which has some odd extensions for templating. (JSONPath itself, of course, isn't very well designed in the first place.)
We're working on a new variant of JSONPath that we're hoping to publish as a formal, comprehensive specification. It's essentially a superset of JSONPath with some syntax warts fixed (like the need to start with $). I wrote a little about it on HN a week ago [1].