When previewing a page, everything on that page will be included in the preview. The end of the preview may have trailing silence removed, however, and the preview time might not match the actual time of the page.
When previewing a track, be sure that at least one beat is activated. Every beat activated will be included in the preview with its own properties. Trailing silence will be removed. The page's properties will also be included in the preview (such as tempo and page filters).
When previewing a beat, only the current beat will be previewed, activated or not. Track and page properties are included. Leading and trailing silence are removed.