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.