fn compress_extended(&self) -> Option<Vec<u8>> {
        self.compress_value_only().map(|compressed_value| {
            let mut res: Vec<u8> = vec![];
            res.push(metadata_byte(
                self.output_size().unwrap(),
                self.operation_id(),
            ));
            res.extend(compressed_value);
            res
        })
    }
Concatenation of the metadata byte (5 bits for len and 3 bits for operation type) and the compressed value.
core/lib/types/src/storage/writes/compression.rs