I'm trying to perform an analysis using the Visibility tool, but something is confusing me about the SPOT parameter. On the ArcGIS Pro website (https://pro.arcgis.com/en/pro-app/2.8/tool-reference/3d-analyst/using-viewshed-and-observer-points-f...) it says that the SPOT is "used to define the surface elevations for the observation points". If I'm using a DEM for my elevation surface, I would assume the SPOT would be the DEM value at my observer point. This fits with the description in this video (https://www.youtube.com/watch?v=q6i9yVm5lsI&t=108s&ab_channel=GIS-Toolstounderstandtheworld) which uses the diagram below. You can see that the SPOT is the height of the terrain under the observer point.
What I find confusing is that on the ArcGIS Pro website, it provides this figure which shows the SPOT extending up to a plane above the observer point. Shouldn't the SPOT value be the height of that brown hill that the observer is standing on, or am I misunderstanding something?