NFMS are the proteins, lactose and minerals of milk. Proteins bind water, help incorporate air and stabilise the structure. That is why a gelato with too little NFMS feels “empty” and melts quickly.
Lactose sets the limit. If NFMS rise too high relative to the water in the mix, lactose crystallises and the gelato turns sandy. In practice, milk gelato sits around 9–11%, depending on the other solids.