What effect this Cap family parameter has on a Structural Foundation family?

In the last several months, I got some emails from customers asking what effect this Cap family parameter has on a Structural Foundation family?

 

This parameter is designed to determine the elevation of the bottom of the footing.

If Cap is selected, the Bottom Elevation will be taken from the "Topmost Bottom-facing Face".

If Cap is not selected, Bottom Elevation will be taken from the "Bottommost Bottom-facing Face".  To see this, refer to the following picture:

 

 From the below picture which is captured from the project file, we can easily find that the value of  "Elevation at Bottom" is different between the "Cap" checked and unchecked.