Double templates extend across both sides of the spread, left and right.
Single templates cover only one side of the spread, left or right.
In Panoramic templates an image/aperture can extend across the spine of the book or album, ie onto both the left and right sides of the layout.
In Standard templates the image or aperture must be on one side only.
Tip: Panoramic templates are available with most types, even those where the print is split at the spine. Just keep critical data (like heads, text etc) away from the gutter (good practice in all albums).