Surflan Pro is a pre-emergent weed control product. Surflan Pro does not control emerged weeds, established weeds, weeds growing from stolens, rhizomes, or root pieces. The area to be treated with this product needs to be weed free at the time of the application. You will need to use 3 fl oz of product per 1-2 gallons of water for 1,000 sq ft. The water in this case is just a carrier and you need to use enough water to make sure you get even coverage and to make sure you are getting the correct amount of product over the 1,000 sq ft area. There are directions on page 7 of the Surflan Pro product label that explains how to properly calibrate your sprayer.

There is no difference between the two.  Surflan AS and Surflan Pro are the same product.  The manufacturer, for whatever reason, call them different things to differentiate between the product sizes, but they are the same product.

Surflan is often used for this type of weed control with great results depending on the type of weeds you are trying to prevent. You will need about 3 fl oz of Surflan per 1,000 sq ft area. Be sure you review the product label to make sure your desirable plants are listed as being tolerant to the product.  Surflan products are known to be pretty thick and will likely clog a hose end sprayer so it is best to apply with a pump or backpack sprayer. The other problem with using the hose end sprayer for the Surflan application is that they are not very accurate and you can wind up dispensing more/less product than needed in a given area which can lead to less than desirable results.  With pre-emergents it is important to apply the right amount of product over an area and water in so that it gets down to the soil to create the barrier.