Every time a unit is published, the previous version of the published unit is saved into the system. The unit history comparison feature can compare any 2 versions from the saved unit history, this includes the current published unit and the current editing unit.
While in editing mode, if there is no published unit for the current editing unit then the history compare button is not active:
While in editing mode, an active "Compare" button is present to compare the current editing unit with the current published unit if a published unit is available:
Clicking on the "Compare" button would bring up the following window that compares the editing unit and published unit side-by-side:
The history comparison feature on the published unit page is almost identical to the behavior illustrated in the section above for the editing unit page except it doesn't automatically select 2 versions for comparison and the button is named "History" instead of "Compare" from the editing unit page.
Clicking on the "History" button will bring up a window to allow you to select 2 history versions for comparison:
Select any two versions by clicking on 2 check boxes and click on the "Compare" button and a side-by-side comparison window would appear:
In the comparison window above, you can change any one of the versions by selecting a different version from one of the two drop-down selection boxes. The window will automatically refresh with the newly selected version. This feature is also available for the comparison window initiated from an editing unit.