Looking at the difference in shapes of the two pads (which is slight) I'm speculating that the upper one may fit a smaller diameter disc but I don't know that. Have you looked at the 'fitment detail' tab to see if there's any info in there which will help you distinguish- different disc diameters etc? Usually there is more than one option due to more than one disc diameter being fitted to a particular model of vehicle (eg, sport and standard spec brakes, or to higher powered versions). Sometimes even different manufacturer's braking systems are used on different spec vehicles or when changes in the production run occur.
Ultimately if you can't find any additional info to help you distinguish then you may need to whip a rear caliper off quickly to study the pad profile- that's what I did with the last set of pads I changed the other day when there were multiple options- took a picture of the shape of one of the old pads to compare with the images on the website. Sometimes there's also a drawing of the pad with dimensions quoted.
