IG, SiSo and SC).
If you look at the solution table you will see that R9C7 and R9C9 are the only cells in R9 which contains the number 1. This means one of these cells must contain the number 1. Therefore we can remove 1 from B9R8 and B9R7. In this case it means R7C7.
The same way you can look at boxes.
If you look in B6 you will see that C9 is the only column where the number 1 exist. Therefore 1 must be in B6C9 and we can remove 1 from B3C9 and B9C9, in this case R9C9.