Not every federation has the same dress policy, but the IFBB is known for its strict guidelines.
Here's the IFBB dress code for men's posing suits. You'll notice it doesn't address the back of the suit. Earlier dress codes did, I believe. There are no rules shown for posing attire in the finals. 1 cm = 0.39 inches.
Rule 24 - Prejudging: Posing attire for all rounds1. Male competitors will wear posing trunks of a solid, non-distracting colour which are clean and decent. Metallic materials, such as gold or silver lame, may not be used to make up the trunks. The use of padding anywhere in the trunks is prohibited. The front area must be covered and the side of the trunks should be 1 cm in width at minimum.