The Shared Edge function will cut the Shared Edges of the Tiled Outside Cuts on the CMC Z1 and Z (9000z) as one stroke between the designs, instead of cutting the outside of each mat individually.
If cutting a single design, it will cut only the Top and Right edges of the outside cut, using the Factory edge for the Left and Bottom, thus not wasting any matboard since the Z1 and Z (9000z) can cut right to the edges.