In Kakuro puzzles there are some clues that can have only one solution if you need to place them in a certain number of empty cells.
     For example:
     In 2 cells:
     3 = 12 (1+2)
     4 = 13 (1+3)

     In 3 cells:
     6 = 123 (1+2+3)
     7 = 124 (1+2+4)

Other clues must have a certain number, and the rest are changeable.
     For example:
     8 = 125 or 134 (1+2+5 or 1+3+4)
     The clue 8 must have the number 1. The other two numbers can be either 2+5 or 3+4.

The Kakuro Cheat Sheet lists the possibilities for all lengths of clues.
The Kakuro Cheat Sheet can also be used for Killer Sudoku.

Enjoy!