State Machines - What happens to 'Don't Care's in the Output?
Truth tables can be used to describe either what a circuit does or what a circuit needs to do.  When a truth table is being used for the former purpose, any particular combination of stimuli would yield output values of high, low, or perhaps "unknown", but an output of "don't care" wouldn't make much sense.  On the other hand, when a truth table is being used for the latter purpose, an output value of "don't care" may be used to invite indicate that a circuit will meet requirements equally well whether the output is high or low in a particular case.