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What is the highest compression level in gzip?

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I see from other sources, including flate2, the highest compression level for gzip is 9 instead of 10. If we pass 10, it should be accepted by parquet but rejected by flate2. Am I getting misunderstanding somewhere?

impl CompressionLevel<u32> for GzipLevel {
    const MINIMUM_LEVEL: u32 = 0;
    const MAXIMUM_LEVEL: u32 = 10;
}