While YAML is easier to process by human beings, JSON has better tool support and is used more widely. In particular, JSON schema is widely accepted as are JSON pointers, and RDBMS such as Postgres have good support for JSON fields as well.
Further, JSON is valid YAML, so YAML parsers can be used for reading the JSON files and the resulting structures can then be validated by JSON validators.
Here, the idea is to increase readability a bit and remove redundancy. The prefix is implicitely defined by the secondary ids on the path to the root.