MFRC522ライブラリのPCD_ReadRegister関数のrxAlignパラメータの機能

私はMFRC522 arduino libary（ github から）をARM MCUに移植しようとしています。 私の問題は、PCD_ReadRegister関数です：

/**
 * Reads a number of bytes from the specified register in the MFRC522 chip.
 * The interface is described in the datasheet section 8.1.2.
 */
void MFRC522::PCD_ReadRegister( byte reg,       ///< The register to read from. One of the PCD_Register enums.
                                byte count,     ///< The number of bytes to read
                                byte *values,   ///< Byte array to store the values in.
                                byte rxAlign    ///< Only bit positions rxAlign..7 in values[0] are updated.
                                ) {

    if (count == 0) {
        return;
    }
    //Serial.print(F("Reading "));  Serial.print(count); Serial.println(F(" bytes from register."));
    byte address = 0x80 | (reg & 0x7E);    //MSB == 1 is for reading. LSB is not used in address. Datasheet section 8.1.2.3.
    byte index = 0;                        //Index in values array.
    SPI.beginTransaction(SPISettings(SPI_CLOCK_DIV4, MSBFIRST, SPI_MODE0));//Set the settings to work with SPI bus
    digitalWrite(_chipSelectPin, LOW);     //Select slave
    count--;                               //One read is performed outside of the loop
    SPI.transfer(address);                 //Tell MFRC522 which address we want to read
    while (index < count) {
        if (index == 0 && rxAlign) {       //Only update bit positions rxAlign..7 in values[0]
           //Create bit mask for bit positions rxAlign..7
            byte mask = 0;
            for (byte i = rxAlign; i <= 7; i++) {
                mask |= (1 << i);
            }
           //Read value and tell that we want to read the same address again.
            byte value = SPI.transfer(address);
           //Apply mask to both current value of values[0] and the new data in value.
            values[0] = (values[index] & ~mask) | (value & mask);
        }
        else {//Normal case
            values[index] = SPI.transfer(address); //Read value and tell that we want to read the same address again.
        }
        index++;
    }
    values[index] = SPI.transfer(0);           //Read the final byte. Send 0 to stop reading.
    digitalWrite(_chipSelectPin, HIGH);        //Release slave again
    SPI.endTransaction();//Stop using the SPI bus

}//End PCD_ReadRegister()

I Read Datasheet several times & can't understand whats function of his parameter! Based on datasheet for reading a register from rc522 :