The value of prtInputIndex corresponding to the input subunit
which will be used when this input subunit is emptied.
The value of zero indicates that auto input switching will not
occur when this input subunit is emptied.
Two different mechanisms for input tray switching,
xcmPrtNextPrtInputIndex and xcmPrtInputAliasGroup, are provided.
A device agent may make use of one or the other, none, or both.
Use the model that fits best for the given application.