The 0.9 x 25 mm (20 gauge) needles are used for smaller animals such as small wildlife, cats and small to medium sized dogs.The 1.2 x 38 mm (18 gauge) needles are used for large animals such as cattle, horses, large wildlife and some large dogs.The 1.1 x 30 mm (19 gauge) needles are a happy medium between the two other sizes, so medium sized livestock and wildlife and some dogs.