The preview tab gives a live web-browser style preview of the current note as it will appear when exported. If the export code uses style sheets, images, etc., these will work in preview if such assets are already exported/stored in the appropriate location within the document's export folder.
The Preview tab can also be used simply for viewing a note in rendered form even if there is no intent to export, e.g. when using Markdown in the actual note $Text.
The Preview pane has been rewritten for v9.5.0 and modernised (to use a WebKit-based control). Note that, while in the past Preview did not write to disk, it now writes temporary files. These files are written to the documents (HTML) export location (as viewed/set via the Export Inspector). Also:
- $Text-embedded images appear in previews.
- Images are automatically exported when a note with images is previewed.