Comprehensive mode is your next step in duplicate detection for your iPhoto library. If you haven't already, it is recommended that you do a simple mode scan first and remove the duplicates that it finds.
Comprehensive mode takes scanning to the next level. Each image is compared to every other image. There's a lot going on, so if you have selected a large number of images you'll have to be patient.
Now because comprehensive mode does all these comparisons, it is irrelevant what sort order the images are presented to ADD. Once comprehensive mode has done it's job, you'll have a full set of date, filename, and hash duplicates ready for you to process for deletion.
Lastly, there are processing options only available from Comprehensive mode. They are:
1. Ability to tag Unique images.
2. Ability to propagate ratings amongst sets of hash duplicates.
3. Ability to propagate comments amongst sets of hash duplicates.