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A subsequence is a sequence that can be derived from another sequence by deleting some or no elements, without changing the order of the remaining elements. On the other hand, a substring is a contiguous sequence of characters within a string. The key difference between a subsequence and a substring is that a subsequence can leave out characters and the order matters, while a substring must have contiguous characters with no gaps. So, a "_" can leave out characters and the order matters, while a "substring" must have contiguous characters with no gaps.