Competition Shirts are often also called show shirts. It is a broad category covering shirts to be worn when competing. The show shirts differ in styles and form depending on the riding discipline. Dressage Show Shirts are completely different in style and form than show jumping show shirts. Even the hunter show shirts are different as well. Equitation show shirts are yet another class of show shirts.
Dressage show shirts usually come adorned with bling and crystals. Show shirts for the jumpers may have some bling as well. However, they are more conservative in general than dressage show shirts.