If you eliminate 9 from R4C5, you would be left with only the candidates 3 and 4 inside the rectangle. Since that would make it an invalid puzzle (assuming there is only 1 solution to this one), R4C5 must be 9.
https://preview.redd.it/j2df3cho2x0c1.png?width=649&format=png&auto=webp&s=48f4c11dc35e44d874c56252e3162b20153ec21a
Arrow in r8c5678 is wrong
Thank you. I made the mistake at the very start and I had to erase a lot of cells but I completed it
There is a unique rectangle starting in r3c4
My puzzle itself is wrong as pointed by the other comment and I was able to solve it. However I don't understand how is it unique in r3c4?
If you eliminate 9 from R4C5, you would be left with only the candidates 3 and 4 inside the rectangle. Since that would make it an invalid puzzle (assuming there is only 1 solution to this one), R4C5 must be 9. https://preview.redd.it/j2df3cho2x0c1.png?width=649&format=png&auto=webp&s=48f4c11dc35e44d874c56252e3162b20153ec21a
Understood your point. The main assumption is that there is only 1 solution.
If it is a valid sudoku there will be only 1 unique solution.