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It will result in a 25% curation reward for the user who voted that comment, and a reward for the person who wrote the comment.

Does the comment have its own reward window? Is it separate from the reward window for the post (that already ended)?

Yes, the reward window is separate at it pays out 7 days after the comment is made.

Now I understand a lot more about the curation reward process. Previously, it deterred me from voting and commenting on posts older than 7 days.