Work offers several different pad or "disk" heights to allow for variance in caliper size across different applications. The L disk is the tallest height meaning that it allows for the largest possible brake caliper clearance, while the A disk is a slightly smaller height to give a larger lip size.
The three setups you listed are basically listed in order of how agressive the fitment is. The 10.5 +27 size will be easy to fit, and will allow a variety of tire sizes with no mofication to the quarter panels or camber settings. It is possible to fit the 11 +25 size in the rear without modifying the rear quarters, but you will need to spec tire size and camber very carefully. Finally the 11 +21 size will more than likely require some widening of the quarter panels to fit.