The app splits one BIP39 seed into two, three or four parts, each of which is an ordinary seed in its own right. Any part can be written down as words or transcribed as a SeedQR. Load the parts back in any order and you get the original. You can also XOR seeds you already hold to derive a new wallet from all of them.
It is N of N, which is how XOR works. Every part is required, no threshold and no spare. A missing part does not throw an error. It quietly produces a different valid seed that opens a real but empty wallet, so the app states the requirement three times before it generates anything, and refuses a duplicate part outright.