Mirrored from Sudopedia, the Free Sudoku Reference Guide
A Hidden Subset is always complemented by a Naked Subset. Because Hidden Subsets are sometimes hard to find, players often prefer to look for Naked Subsets only, even when their size is greater.
In a standard Sudoku, the maximum number of empty cells in a house is 9. There is no need to look for subsets larger than 4 cells, because the complementary subset will always be size 4 or smaller.
Let's take a Hidden Pair as an example. When 2 digits can only be placed in 2 cells in a house, these 2 cells are completely reserved for those digits. The remaining candidates in these cells can therefore be eliminated.
You can also look at it from another perspective. What would happen if we would place one of the other candidates in one of these cells? The result would be that there is only one cell left for the 2 digits that belong to the hidden pair. It would be impossible to complete the puzzle.