The Associate Edge to Surface option allows you to associate the edges of blocks to surfaces. The color of the selected edges will turn white/black, indicating that the nodes on that edge will be projected to the nearest active surface. By default, edges and faces between two blocking materials (boundary faces) will automatically associate with the nearest active surface. Related vertices and edge splits will also be associated with the nearest active surface. When moving vertices or edge splits, there will be no movement until the cursor passes over an active surface. Ensure that the vertices or edge splits are moving on the correct surface.