A page template is a complete proposal layout: loading it replaces everything on the canvas. A section template is a reusable fragment (for example a testimonial band or a closing call-to-action): loading it appends the fragment to the end of the current canvas without removing existing content. You choose this in the Save as Template pop-up's Type dropdown when using Save Current as Template -- the options are labeled Full Proposal (page) and Section / Block Group (section). Templates saved with the editor header's Save Template button are always saved as full-page templates.