It refers to all the elements and figures of a map exteriors to the charted surface“ (definition from CFC, 1990)
To become a complete map, a geographic image needs to be layout with exteriors elements, who are helping to the understanding of it (title, scale, legend, etc.).
Source : LAMBERT Nicolas, ZANIN Christine, 2016, Manuel de cartographie, Armand Colin., s.l., 221 p.