The Units per Pixel-value (UPP) is the resolution of your input data and it affects the rendering quality of your source.

If you are combining a Height source with an Imagery source, you don’t have to add a UPP-value because DM reads the resolution value from the Imagery source. But what if you have height data but no images? You can still configure a Height source and combine it with a Color source instead. In that case you will have to add a value to the Color Source because the Map Dataset will always use the UPP-value from the Imagery sources and not the Height Sources. The UPP-value on your Color Source will affect the resolution and rendering quality of your height data.