Scan Selection does a full and comprehensive scan of only the images you have selected within Aperture or iPhoto, and performs ImageIQ on potential duplicates.

It is irrelevant what sort order the images are presented to ADD. Once complete, you'll have a full set of date, filename, and hash duplicates ready for you to process for deletion.

Lastly, there are a few advanced processing options that might interest you. 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.