The cells C1, A2, C2, A3, B4, B6 and B9 can together accept various combinations of values. But some combinations of values can be excluded, because they would leave some cells with no potential values.
More precisely, the following combinations of values are not possible
for the cells C1, A2, C2, A3, B4, B6 and B9:
0, 1, 0, 1, 0, 0 and 0 because the value can occur only once in the region
5, 9, 6, 1, 7, 8 and 4 because the cell B1 must contain 1, 8 or 9
5, 9, 6, 1, 7, 9 and 4 because the cell B3 must contain 1, 4 or 5
5, 9, 6, 1, 7, 9 and 8 because the cell B1 must contain 1, 8 or 9
5, 9, 6, 1, 9, 8 and 4 because the cell B1 must contain 1, 8 or 9
5, 9, 6, 1, 9, 8 and 7 because the cell B1 must contain 1, 8 or 9
6, 9, 5, 1, 7, 8 and 4 because the cell B1 must contain 1, 8 or 9
6, 9, 5, 1, 7, 9 and 4 because the cell B3 must contain 1, 4 or 5
6, 9, 5, 1, 7, 9 and 8 because the cell B1 must contain 1, 8 or 9
6, 9, 5, 1, 9, 8 and 4 because the cell B1 must contain 1, 8 or 9
6, 9, 5, 1, 9, 8 and 7 because the cell B1 must contain 1, 8 or 9
8, 9, 5, 1, 7, 8 and 4 because the cell B1 must contain 1, 8 or 9
8, 9, 5, 1, 7, 9 and 4 because the cell B1 must contain 1, 8 or 9
8, 9, 5, 1, 7, 9 and 8 because the cell B1 must contain 1, 8 or 9
8, 9, 5, 1, 9, 8 and 4 because the cell B1 must contain 1, 8 or 9
8, 9, 5, 1, 9, 8 and 7 because the cell B1 must contain 1, 8 or 9
8, 9, 6, 1, 7, 8 and 4 because the cell B1 must contain 1, 8 or 9
8, 9, 6, 1, 7, 9 and 4 because the cell B1 must contain 1, 8 or 9
8, 9, 6, 1, 7, 9 and 8 because the cell B1 must contain 1, 8 or 9
8, 9, 6, 1, 9, 8 and 4 because the cell B1 must contain 1, 8 or 9
8, 9, 6, 1, 9, 8 and 7 because the cell B1 must contain 1, 8 or 9
Because some potential values of A3 occur in none of the remaining combinations, they can safely be removed.