It's a computer simulation of cartridge performance with databases including many cartridges, bullets and propellants. You need to enter cartridge, bullet, barrel length, cartridge overall length and propellant type/quantity to calculate an estimated maximum chamber pressure, muzzle velocity, and other data.
Here is a screen shot of my load but with the default COAL. Don't recall my COAL off hand, but it's longer which makes my calculated pressure a little lower than shown here: