RDM = reduced diameter mast
STD = standard diameter masts
These are the two types. The RDM masts are super tough and the norm for almost all sails below 6.5m. The STD masts still work better with larger sails, especially with cambers.
Not all mast brands have the same bendcurve and stiffness ratings and the modern sails are specific in their requirements. So best to ask us what works the best as it’s the single most important and complicated relationship in the whole windsurfing setup!!!!!