Foil Promo Packs have the same assortment of cards as regular Promo Packs, but all the cards are foil.
Typically, you will receive 75% regular Promo Packs, which contain non-foil cards, and 25% Foil Promo Packs, which contain all foil cards. This division may change from time-to-time and will be announced.
Foil Promo Packs are labeled as such on the outside of the packaging.
You are free to use Foil Promo Packs as you would use regular Promo Packs at your discretion. Some retailers choose to reserve these as top prizes for events or to cater specifically to crowds that prefer foils (such as Commander players).