When
optimizing a
digital system, if a given
input cannot occur, we "don't care" what the
output is for that
input.
For the purposes of simplification, we can make the "don't care" output be whatever makes the system simple; it will not affect the proper operation of the system.
On a Karnaugh map, "don't care" is usually represented by a 'D' or 'X', and can be counted as either a 0 or a 1, depending on which makes the system simpler.
See also Quine-McClusky method.