The fill valves that come from the factory are set at the proper refill percentage for that toilet. The "refill percentage" is the percentage of the water flowing through the fill valve that is diverted into the little hose between fill valve and overflow riser, and which then goes down the overflow riser to refill the bowl so that it has water in it with which to start the flush.
For the original Ultramax, which is a GMax flush, the proper refill percentage is 40 percent. The original valves that came in the toilet from Toto, regardless of valve manufacturer, were set at 40 percent. Most of them were Korky, but they also installed a Chinese valve of Toto design, called the Voreto valve (for its manufacturer, Voreto (Xianen) Manufacturing, Ltd., a partnership with Toto). The Korky 528s with the blue top, the ones that you can buy in the store labelled "Fits Toto", the 528T model, are also set at 40 percent.
Later Totos, using the EMax (1.28) flush, had a different refill percentage, and the valves from the factory were set to the proper refill percentage for them.
The newer Toto designs, called the II Series, include the Drake II, the Ultramax II, the Vespin II, the Carlyle II, etc. These all come with CEFIONTECT as a standard feature, a Universal Height bowl, and a 1.28 gpf flush that has no vertical holes in the bowl rim, but rather some openings that spray horizonally to create a swirl ("cyclone") rinse. I don't think it makes the flush any better, but it gives a little better bowl rinse, particularly in combination with the CEFIONTECT. All these toilets have a 28 percent refill ratio, I believe, although it may vary between model. The valves that come in them, whether from Korky, Voreto, or another manufacturer, are preset at that refill percentage.
With all these refill percentages, Toto commissioned Korky to create, manufacture, and supply Toto's Universal Toto Replacement Fill Valve, model TSU99A.X. It's basically a Korky 528MP (MaxPerformance) fill valve, which has a little knob right where it connects to the refill hose that lets you vary the refill percentage from zero to 40 percent. The Korky-branded 528MP has instructions on how to guesstimate the proper refill setting. The Korky-made-but-Toto-branded TSU99A.X comes with a little gizmo that lets you set it precisely for your model of toilet. That's the major difference. The TSU99A.X allows Toto and its supply-house partners and plumber partners to stock ONE Toto-branded replacement fill valve that will work on any Toto gravity toilet, rather than having to stock replacement parts for each one.
Me, personally, I like the Korky 528 design (and the TSU99A.X) because once it is installed, all the parts that will wear out (the "wear parts") are contained in a little cap that you can get at Home Depot for $4, and thus can fix it when the time comes without having to take the valve out of the tank. You just pull off the top, access the cap, pull out the little strainer inside and rinse it, put it back in place and put in the new cap, put on the top, and you're in business. Just that one cap (Model R528) fits in every Korky 528: the white-top basic 528, the red-top 528Z with leak-guard, the blue-top Fits Toto 528T, the silver-top 528MP and the black-top 528PRO, as well as any Toto-branded 528, including the TSU99A.X. Takes me less than a minute to change that cap. I also like the Korky 528 because the Voreto valves can suddenly start screeching or chirping on shutoff. I ripped them out of our house toilets and replaced them with the 528T where appropriate, and the 528MP in others. Aaaah, back to silence.
Terry has said in other posts that he likes the 528MP because it's the only fill valve he has to carry on his truck (he now uses a plumbers-only variety of it called the 528PRO) to fix any Toto fill valve. He also says that it allows him to just tell customers over the phone how to fix their problems by replacing that little cap, instead of coming out to their house. (Of course, the good folks at Korky are always willing to provide over-the-phone guidance on replacing or servicing any of their products, and their 800 number is staffed by nice people in Wisconsin, where they manufacture their stuff. They also have links to YouTube from their web site, where they have posted semi-professional-looking videos of how to install or service most of their products.)
Let us know if you have any other questions!