Medium Boy
Medium Boy fashion dolls are more slender than Large boys, but more muscular than Slimline. I think of this as a swimmer's build type of body.
Dolls that can be clothed with these patterns at 100% are Ever After High boy dolls, Disney Descendant and prince boy dolls, Fashionista Slim (I know it's confusing, they came out after I named my doll sizes) Ken dolls.
At 108%, they fit the new "Original" Fashionista Ken size, but some patterns marked "Original edition" have been pre-resized so you don't have to enlarge them to sew.