In the classroom or on a test, these should be given or implied in the problem. Let’s think about a real world example though. Let’s think about using it for an input. Let’s say we are working on a burglar alarm system in a shop and the inputs ABC corresponding to The discrete signals “Arm Alarm”, “Door alarm”, and “motion sensor”. The combinations of 111, 101, and 110 return an output of 1 (or sound alarm). The combination of 100 results in a 0 output (no alarm). What about the remaining 4 cases in the truth table? They can be quickly described as 0dd and they correspond to an output of 0, right? When we use don’t cares in the outputs it usually corresponds to combinations of inputs we don’t expect to see. A good example would be turning on a left and right turn signal at the same time... by adding don’t cares, we can (Hopefully) make larger groups and reduce our input requirements to our gates. The result of this is less reliability If a forbidden input state is entered. However, you just made your circuit much simpler and cheaper by using less or simpler components.
