In classical logic, two propositions are said to be in subalternation when the truth of the first proposition (the superaltern) implies the truth of the second (the subaltern), but the reverse is not true.
1. All dogs go to heaven.
2. Fido went to heaven.
Knowing that all dogs go to heaven is sufficient to tell you that Fido went to heaven; knowing that Fido went to heaven is not sufficient to tell you that all dogs go to heaven. Furthermore, if you can disprove the statement that Fido went to heaven, you have also disproved the more general statement that all dogs go to heaven, but the reverse is not true.
In classical logic this was one of the four relationships possible between propositions, as observed in the square of opposition. The others are contradictory, contrary, and subcontrary.
Brevity Quest 2016