The suit that forms the lower trump cards. All Diamonds (A♦ through 7♦) are trump, ranked below the Jacks.
The powerful suit that beats all fail cards. In Sheepshead: all Queens, all Jacks, and all Diamonds are trump (14 cards total).
Any suit that is not trump. In Sheepshead, Clubs, Spades, and Hearts (without their Queens and Jacks) are fail suits.