The Custom Layer terrain tool is available to Explore members on both the FATMAP app and website. It's located in the terrain tools menu on the bottom right-hand side of the screen.
Using this feature, you are able to set your preferred settings for Elevation, Gradient and Aspect. Once you have set these and tapped Apply, the areas of the map that apply to these settings will be highlighted in green.
There will also be a key at the top of the screen to show you the settings you have selected. You can go back into the Custom tool at any time to either change or reset these values.