The highest version of SXP protocol that this device supports.
'unknown' - The SXP protocol version capability for the
device is unknown.
'one' - The device supports SXP protocol up to version 1.
'two' - The device supports SXP protocol up to version 2.
'three' - The device supports SXP protocol up to version 3.
'four' - The device supports SXP protocol up to version 4.