Specifying the path to an HTML mark-up processor in $HTMLPreviewCommand results in the ^text^ command (only) being processed for HTML mark-up using the alternate processor. This allows the user to choose a Markdown or any other markup processor. The output is used both in the text Preview pane and HTML export.
A simple example is to specify the path to an installation of Markdown. This then allows Markdown-Style mark-up to be used in $Text and interpreted as such during evaluation of ^text^ for HTML export. Whereas normally italic text in $Text would be exported as <i>italic text</i>, a Markdown user would use '_italic text_' in their $Text instead.
See Markdown preview rendering for available values for $HTMLPreviewCommand.
If not empty, ^text^ passes the unprocessed text of the note to the script in $HTMLPreviewCommand instead of processing the text itself.
The 'Markdown' built-in prototype, presets notes to use Tinderbox's built-in copy of the Markdown script for easy configuration. More expert users may chose to edit this to point to their own copies of Markdown, or other such scripts.
See also—notes linking to here: