Currently the unit seems to be:

• kilometers for Lucene (see https://hibernate.atlassian.net/browse/HSEARCH-1872?focusedCommentId=82458&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-82458 , the line starting with "Should we keep in(Unit)")
• to be determined for Elasticsearch (see HSEARCH-2326 Open )

We should at least support the metric system units (which seems to be the default in both backends). Don't know if imperial system units support would be useful. Maybe we should see what Elasticsearch does in this regard.

The customization support should be added either:

• only for the Elasticsearch backend
• or for both the Elasticsearch backend and the Lucene backend.