This is a pyang JSON Schema output plugin. It takes YANG files and tries to produce a JSON Schema that validates JSON content similar to the payload as defined in RFC7951.
Here's an example when running from the test directory:
$ pyang --plugindir ../ -f json-altschema ./test-module.yang > test-module.jsonschema
It is a derivatin of the json-schema plugin. It is ultimately intended to produce less schema thatn the original plugin AND also map the schema to a form that can be used in Swagger.