What ${source_elasticsearch} is in Elasticsearch docker file

in Elasticsearch docker file: https://github.com/elastic/elasticsearch/blob/7.1/distribution/docker/src/docker/Dockerfile

I found line 25: ${source_elasticsearch} what is this in docker file? why need this ? how does it used and worked?

in line 27: RUN tar zxf /opt/{elasticsearch} where is {elasticsearch} defined?