The calculator works out partial-pressure blends of nitrox and trimix with the real properties of oxygen, helium and nitrogen. Blend to a target tells you how much of each gas to add and what the gauge reads after each step. Top up gives the mix you end up with when you add gas to a cylinder. Continuous blending what the analysers at the compressor intake should read. With a cylinder size entered, every tab also gives the mass and free gas of each gas added.
The simple partial-pressure rule treats every bar as the same amount of gas. At cylinder pressures it isn’t: a bar of helium holds noticeably less gas than a bar of oxygen. A real fill can miss the simple recipe by a few bar, so the calculator shows both.