As far as most serious luthiers are concerned there are 3 grades of wood. A, AA, and AAA. The reputable suppliers and luthiers stick to this grading system. Though truth be told, I don't know too many luthiers who's work I respect that make a big point of what grade the wood in a particular instrument may or may not be. They know that it has very little to do with how the instrument plays and sounds once you get to the mid grade of AA.
Once in a blue moon there will be a set that will come through that will be so exceptional that a supplier may designate it as Master Grade. I've watched over the years the number of sets I see coming onto the market designated at Master (or now 5A) from being virtually non existent to so commonplace that one is left wondering if there is any "plain old tone wood" available any more. You can make your own conclusions here.
The difference in grading varies on wether you are talking softwoods like spruce or cedar. Or hardwoods that are usually used for backs and sides in guitars, but can be a top as well in ukes. With the softwoods it has to do with runout, how well quartered it is, grain spacing, stiffness and colour. Once you get to AA+ the differences pretty much just come down to the aesthetics. With AAA being a homogenous even colouring, but many times no better as far as building a great sounding instrument than the lesser grade AA.
Hard woods have some of the same criterial as well. Depending on the species you get different figure if it's quartered or riff sawn. Free from insect damage. Good colour, but depending on the species grain spacing is much less of a concern. Figure and rarity does come into it, and while really wild curl is nice to look at, it's a rare piece of that wood that will make a great instrument.
So the long and short of it is that the term "Master Grade" or 5A or whatever the next catch phrase is should be taken with a very big dose of salt. It's being bandied about by everyone and their dog as nothing more than a marketing catch phrase in an attempt to part the gullible from their $$$ in my opinion.
I won't even tell you what grade the wood I use in an instrument is as it just doesn't matter. You either like the look, the sound and the playability, or you don't.