Howdy Roger. Welcome to the forum. I use half the nutrient called for after the ferment gets going good. About a third of the way thru the ferment, pitch the other half in. If the ferment vessel is full when the ferment is going good, the addition of more ingredients may cause it to foam up more and come out of the ferment vessel. Guess how I know. LOL, Arne.
DAP (diammonium phosphate) is a source of nitrogen for your yeast, which it needs to conduct alcoholic fermentation. It is a good source of nitrogen. There are other micronutrients which are also beneficial to the fermentation process, and so there are also other nutrients which are available to us as winemakers.
Many of us here use Fermaid K and Fermaid O, the K contains some DAP, as well as other micronutrients, the latter contains no DAP and is organic. Typically the nutrients are applied in two stages, 1/2 of the recommended dose at the end of the lag phase (when yeast have multiplied and begin to actually ferment), the second 1/2 when roughly 1/3 of your sugar has been reduced to alcohol.
I stull use DAP sometimes, but only for the initial feeding just after lag phase, and switch to Fermaid K for the second 1/2 dose. Some folks here use nothing but K, some nothing but O, so you'll see some varying recommendations.
If you are doing kit wines and using the supplied yeast, the nutrients that are necessary to ferment are generally already in your juice. Hope that helps!!