Unlike other mods that make inserters faster, but still have them move 12 items at a time, this mod gives 3 direct upgrades to allow inserters to move full stacks of 50, 100, and 200 items in a single swing.

Due to how inserters are coded within Factorio's C++ code, no mod can increase the base capacity of an inserter, meaning it is mechanically impossible to create a new inserter item for you to build with a different capacity than an existing inserter of the same type (there are two types, standard and stack). Thus, all stack inserters of any variety will always have the same capacity per swing. That is why almost all mods focus on increasing swing speed.

As for the discrete capacity upgrades? If you have a rail car with 10 stacks of Iron Ore, and are using this mod at the 200 upgrade level, you'll notice that only 50 ore are moved per inserter at a time. This is, again, because of how inserters are coded within the C++ of Factorio's code. The inserter can only ever pick up the <stack size> of a given item type. If the stack size is 50, then a 200-capable inserter can only pick up 50. No mod can change this.