A don't-care entry says the specification permits either output for that input combination. In minimization, use it as zero or one only when that helps simplify the required behavior. It is not permission to ignore an input that can occur and needs a defined safe response.
Worked example
Let F(A,B,C) have ones at 1 and 3, with don't-cares at 5 and 7. Treating those don't-cares as ones forms a four-cell group with C = 1. The simplified result is F = C, while all specified zero rows still produce zero.
What to watch for
Unreachable input assumptions must be justified. A supposedly invalid state may occur during reset, a fault or an external input transition.
Try it yourself
Must every don't-care cell be included in a group?
Answer: No. Include only the ones that help. The minimization must satisfy required zeros and ones; the don't-care outputs remain unconstrained by that specification.
How to study this topic
Keep the original truth table beside your simplified result. Check required ones and zeros separately, and document any don't-care assumptions. A smaller expression is useful only when it still matches the specification.
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