The "Include Changed Projects Only" checkbox serves as a filter for the Simple Reports tab. By selecting this checkbox, only projects that have pending or approved changes within the selected Revision will be returned. If the STIPs selected for comparison do not track Revisions (such as a Future STIPs or Scenarios), this checkbox will be deselected and disabled.

Where is it located?  

On the Reports module, the landing page (i.e. the Simple Reports Tab) has this checkbox at the bottom right corner of the interface under the "Output" title. 

What does it look like?

The table below encases the checkbox option in a red rectangle to show both its format and location.