In a perfect world, a mash, when brought to a rest temperature, can be held at that temperature for the duration of the rest. If you are using a mashing system that can maintain heat (RIMS, Direct Heat, Steam Jacketed, etc.), then you actually can maintain a constant temperature throughout the entire rest, and this section need not apply to you. For the rest of us, who infuse at a given temperature and then hope the temperature holds, a variety of factors come into play; ambient temperature, mash tun design and material, etc. Furthermore, if you are doing decoction mashing, and are not able to bring the main mash to a constant temp, the act of pulling the decoction may drop the temperature of the main mash considerably.

Therefore, it is extremely critical that you become familiar with your mashing system and it's temperature losses in order to accurately predict multi step infusion and decoction amounts and temperatures.

For instance, look at the following dialog and chart from the mash designer, which shows the rests having the same start and stop temperatures (a perfect world):

In reality however, the brewer may lose heat during the rests. Pulling decoctions will also drop the temperature (perhaps drastically), and could have a big impact on the correct amounts and temperatures predicts. ProMash uses the Stop Temperature of a rest to predict the infusion temperatures and amounts of the following rest, as illustrated below :

Knowing your system is the only way to properly account for these heat losses, and it is crucial you do know them (if they occur) and to enter them accordingly in the ProMash mash schedules. This may take some trial and error until your become intimately familiar with your mashing system, but is necessary for accurate predictions.