What IS the difference between size 6 and 6x, in little girls clothes?

OK, I'm confused, what the heck is the difference exactly? Does someone know the answer to this question? I've asked numerous moms and none of us seem to know what the answer really is. Is 6x bigger? wider? longer? all of the above? The sizes don't seem different to me, if I hold up these items in the store next to each other or on top of each other, I don't see it. Why the 'x'? Does it continue into higher sizes? I guess I haven't noticed since my daughter wears a 6 currently so I haven't paid attention to the bigger sizes yet. Someone please enlighten me :D