The cells H3 and I3 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 H3 and I3:
8 and 4 because the cell G3 must contain 4 or 8
8 and 5 because the cell H2 must contain 5 or 8
8 and 7 because the cell A3 must contain 7 or 8
8 and 8 because the value can occur only once in the region
Because the value 8 of the cell H3 occurs in none of the remaining combinations, it can safely be removed.