When a region has only one empty cell, it takes the missing digit.
Where to lookA pointer first: look at row 2. It is one digit short.Full houseOnly row 2, column 4 is empty in row 2, and the digit missing is 1.
Look for a row, column or box missing just one digit. Work out which digit it is and enter it in the empty cell. On boards with diagonals or windows, those regions work the same way.
This example, step by step
Where to lookA pointer first: look at row 2. It is one digit short.
Full houseOnly row 2, column 4 is empty in row 2, and the digit missing is 1.
In hints
When stuck, tap "Hint". For a full house it points to the region, then marks the empty cell and the missing digit.