I used a step-by-step Sudoku solver for a puzzle in which I was stuck.
However I don't understand how the next number (6) was found.
Sudoku puzzle where I was stuck:
Sudoku puzzle solved by one step forward:
The solver says the only candidate for cell (R4, C5) is a 6. Why couldn't it be in cell (R6, C5)? I need a clear explanation; I'm not a Sudoku expert.