MMA BOARDSHORTS are the ultimate crossover training shorts because they are Light, fast drying and made of comfortable microfiber fabric. There is a difference between Muay Thai Shorts and MMA Boardshorts. The main difference is the lack of metal components on MMA shorts compared to Board shorts. Things like metal eye loops for drawstrings and zippers are not kosher for grappling as they can result in cuts and other injuries. In fact, some grappling tournaments ban boardshorts for this reason unless you can somehow prove your shorts are safe to roll in for your opponent.
Most MMA shorts lack exterior pockets which is another safety concern being that someone's finger could get caught in it also resulting in injury. MMA shorts are also designed to stay on while rolling and tend to have very secure waistbands which include a lace drawstring AND a velcro flap over it.
Also some MMA shorts sport stretch panels in the crotch and slits on the outside of the leg. The stretch panels are great for flexibility and to some, provide extra friction for their guard. The leg slits are for kicking, and to a certain extent the stretch panels too, is something board shorts are not designed for.