A Buddy Cell
is any cell that is in the same units
(row, column or box) as the cell you are working on. The content of a cell will influence all it's Buddy Cells.
Each cell in a 9 x 9 Sudoku puzzle has 20 buddy cells.
In Figure 1 all the Buddy Cells of the red cell in row 4 column 8 are yellow.
Any cell cannot have a value in it that is already placed in any of it's buddy cells.