https://github.com/Atmosphere-NX/Atmosphere/blob/master/docs/features/cheats.md
The register does NOTHING, unless another cheat writes anything to it.
For Code Type 0x0, the register is "read only", nothing is written to it, so both codes work correctly no matter what register you choose.
Code:
[test1]
040F0000 00123456 00654321
[test2]
040F0000 00654321 00123456
translates to:
Code:
write32(Main + 0x0000123456 + r15, 0x654321);
write32(Main + 0x0000654321 + r15, 0x123456);
both codes work without fail, because r15 is by default 0, but if you write a code like this:
Code:
[test1]
040F0000 00123456 00654321
[Master Code On]
400F0000 00000000 09999999
[test2]
040F0000 00654321 00123456
then it would translate to this:
Code:
write32(Main + 0x0000123456 + r15, 0x654321);
write_reg32(r15, 0x0000000009999999);
write32(Main + 0x0000654321 + r15, 0x123456);
the first code would work correctly, the second would not, and all codes that use r15 after the master code would fail.