Manual S is a method of selecting the proper equipment to match the heat load requirements of a home. It takes into account the sensible and latent loads of the house and manufacturers data for actual capacity to properly match the equipment you are using for the home. It takes into account the staging of the equipment and its ability to step up or down to control humidity. It no longer just considers the size of the equipment but different capacities and efficiencies.
With the rise of tighter builds and more efficient homes comes more challenges for Hvac equipment, it needs to be able to cool the house when it is 100 degrees and still run long enough to pull humidity when its 80 degrees. The one size fits all method is no longer a smart way to select equipment if it ever was. The efficiency of buildings makes the single stage air conditioner severely lacking if undersized to accommodate low temp humidity problems yet just as troublesome when sized large enough to cover 100 degree temps. Manual S has limitations built in for single stage, dual stage and variable speed HVAC equipment to ensure that you get as close of a sizing as possible. Once you have the Manual J and the S done you're ready for the final calculation for your home comfort, the duct sizing and Manual D.