We just released Elasticsearch 6.3, which includes the Basic features of X-Pack by default, with no time limits and no need to register for a license.
This features are released under the Elastic License. It would be best for you to read those terms yourself, and if you have questions then feel free to ask us, or you can seek your own legal advice.
We also provide documentation and FAQ about the license.
If you do read those license terms, you will find that they are focused on providing you as the software user, the right to make use of our X-Pack features within some appropriate limitations. They do not seek to grant special additional rights or privileges to Elastic.
Note, this is not legal advice. You should read the terms for yourself, and then seek whatever legal advice you feel is necessary.
There is no obligation for you to register with us, and no way for us to know who is using Elasticsearch with X-Pack unless you sign up for a paid subscription or let us know in some other way, so we would have no way to use your company name for promotion activity.
If you wish to use X-Pack Basic on an earlier version of Elasticsearch, then the license terms will be different, and you should consult the license that applies to the version you are interested in using.