The Set Vertex Mapping command enables you to set groups of vertices to have the same vertex-mapping coordinate.You must choose the u (horizontal) or v (vertical) coordinate and give it the new mapping coordinate required.
For example suppose you want a square mapped so that the map is stretched at the bottom and squeezed at the top, one way to do it is to create a rectangle and subdivide it once so that it has 9 vertices, shown on the left below. Then use a planar map to apply mapping coordinates to the vertices, shown below on the right.
Select the 3 vertices in the middle row, and modify their Vertical mapping coordinate to a Value of 0.3 to give mapping coordinates.
When rendered the map will appear stretched in the bottom half of the rectangle and squeezed in the top. It's horizontal appearance is unaffected.