Unit or Container
or Container is a group of cells that are related on the Sudoku puzzle. A unit is either one row, one column or one box in a Sudoku puzzle.
A Sudoku puzzle can have one and only one of each value in each unit.
In Figure 1 the grey cells show a row unit, the green cells show a column unit and the red cells show a box or block unit.
Once a puzzle is completed there has to be one of each value in each unit. There cannot be more than one instance of any value in a unit and each unit has to contain all values.