The option in the Export Data panel to “Include Prefixes” has no effect, regardless of its setting. Prefixes are always included with HTML, plain text, and Markdown, and never with OPML.
In the case of OPML, exported data loses all of its item types when re-imported. Manually adding prefixes to the OPML file works, i.e, items are re-imported with their correct types. But there’s no way to export them. Exporting the prefixes could also help with importing to other outliners that support corresponding item types.
