Look RS232 > RS232C Hardware  
  Scratch Register

The scratch register is not used for connection, it is rather used as a place to leave a data byte. The only usage which is not meaningless is possibility to find out whether the UART is a 8250/8250B or a 8250A/16450 and even that is not very practical today as the 8250/8250B was never designed for AT's and can't hack the bus speed.

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