Prior to Elastic 6.3, X-Pack was an extension of Elastic Stack that could have been installed on top of Elasticsearch or Kibana. Now, Elasticsearch and Kibana come in two flavors:
- OSS version, that is, Apache 2.0License
- Elastic license
If X-Pack isn't available with the OSS version, then you have to download Elasticsearch and Kibana with the Elastic license. When you download Elasticsearch or Kibana with the Elastic license, all the basic features are available for free by default. The paid features can be used as a trial for 30 days; post that time period, a license has to be bought.
To see a list of all the features that are available for free and the features that are paid, go to https://www.elastic.co/subscriptions.