Document: Will the "Edge" snapping environment create coincident lines or polygons?
Created: 10/13/2004 (Tom Evans)

When you are digitizing line or polygon features and you set the snapping environment to snap to "Edge" (checking the "Edge" box in the snapping environment), your newly created vertices (for the line or polygon feature you are creating) will be snapped to any location along a line/polygon feature within the snapping tolerance.

However, the line/polygon-edge you create will not have the same line morphology because the number and location of vertices in your new digitized line are not in the exact same place as in your snapping dataset.

For example, assume you are creating a zoning data layer and you want the edge of this data layer to be coincident with a parcels data layer which already exists. If you set snapping to "Edge"s in Parcels, the vertices in your zoning layer will indeed be located on edges of polygons in the Parcels data layer. But the line you create from those verticies will not coincide with the edge in the Parcels data layer. See this figure.

Note that with this method, the lines you create in Zoning are not shared with Parcels and so any modifications to the Parcels data layer will not change the Zoning data layer (something you would probably want to happen).

The proper way to create coincidence is to use either Map Topology or Geodatabase Topology (for feature classes in the same feature dataset within a Personal Geodatabase).