In the rows B, F and G, the only examples of candidate 6 are to be found in the blue highlighted cells. We can't tell exactly at this stage where the solution 6s will be, but clearly there must be one in a blue cell in each of the rows B, F and G, and they will be located in such a way that they simultaneously provide a solution 6 in each of the columns 5, 6 and 9. Any other candidate 6s in the columns 5, 6 and 9 (the ones highlighted in magenta) may be removed.