The Decoction Volume page is used when performing a decoction mash. A decoction mash is where the various step temperatures are achieved by removing part of the mash, boiling it in a separate container before using it for infusion water to heat the remainder of the mash. It is traditional in many continental European beer styles and the bottom line is you get a much better malty profile in your beer. There are some great articles out there on it if you’re looking for a little something new to put in to your mashing routine. A quick search on your favorite search engine should give you plenty of articles on it.
What the Decoction Volume Page allows you to do is determine how much of the mash at what “Mash Boil Temperature” you need to take out and heat up in order to reach the “Target Mash Temperature”. So as an example let’s say you’re mashing a “Starting Water Volume” of 12 quarts for 10lbs of grain and your “Starting Mash Temperature” is 145 F. You think your “Target Mash Boil” temperature is 210 degrees. You know that you need to extract(Volume To Decoct) 2.09 quarts from your mash and heat it to 210 degrees F in order to add it back to your mash to achieve a “Target Mash Temperature” of 154 F.