It could be considered both. You replaced the downed fence, which would be a repair. But you replaced it with a better, more durable product, which could be considered a home improvement. Are you asking this for a specific tax-related question?
Most of the time, home improvements are not tax deductible, except in the way that the improvements increase the value of your home. That would come into play if/when you sell your home by raising the cost basis in your home.
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