The number of decoctions a brewer chooses on depends on how many hot water infusions will be used for various rests called out in the recipe. Depending on the style, there may be four or five temperature rests. A single decoction is generally used to reach mash out temp of 172F. Mostly liquid is removed from the mash and boiled in a single decoction. A double decoction would use the 1st decoction to raise the main mash temp up to conversion temp and the 2nd decoction would be used for mash out. In a tri decoction the 1st decoction can be used for raising the main mash temp up to a beta rest. The 2nd decoction can be used for reaching alpha temps in the main mash and the 3rd decoction would be used to mash out after conversion. In the tri-decoction method, each decoction can be heated to conversion temperatures different than main mash conversion temps. Going old world, the beer would have been brewed using tri-decoction method. There might be a rest somewhere between 95 and 105F, another rest somewhere between 120 and 135F, maybe there are two beta rests at 145 and 149F, an alpha rest somewhere around 155F, then, mash out. Brew On!!!