EDIT: NVM.. I didn’t comprehend the original reply. It wasn’t on height map but feed rates.
Height mapping shows the Z height differences between areas of the work surface.
For instance, I have a homebuilt CNC using GRBL and an arduino.
Unfortunately, the bottom left is about 0.6mm higher than the top right.
A height map would compensate for this.
This is most commonly used in the 3D printing world as auto-level.