When $PRINCIPAL has different input/output devices, the USE command recognizes intrinsic special variable $ZPIN to apply appropriate deviceparameters to the input side of $PRINCIPAL. A USE with $ZPIN sets $IO to $PRINCIPAL for READs and WRITEs from the input and output side of $PRINCIPAL. $ZSOCKET() also accepts $ZPIN as its first argument and, if the device is a split SOCKET device, supplies information on the input SOCKET device. In any context other than USE or $ZSOCKET(), or if $PRINCIPAL is not a split device, $PRINCIPAL, $ZPIN and $ZPOUT are synonyms. In the case of a split $PRINCIPAL, $ZPIN returns the value of $PRINCIPAL followed by the string "< /" Any attempt to OPEN $ZPIN results in a DEVOPENFAIL error.

For more information refer to “$Principal”, “$ZPOUT”, and “$ZSOCKET()”.