'I/O Line Passing' is a feature that allows output(s) of a remote node to replicate Digital or Analog input(s) on local RF device. Many Digi XBee devices including 'Digi XBee SX 900' modules support this feature. Check below link from product manual of module for more information:
However, users must note that although 'Digi XBee SX Modems' contains a 'Digi XBee SX 900' module inside, it does not
support the 'Line Passing' feature. That's because as per design of this feature, inputs are bound only to the same pin on remote device as outputs. However, internal circuitry on modems does not invert I/O line & rout it to same output pin on hardware making it impossible to support this feature.
Sep 30, 2020