$ZIO contains the translated name of the current device, in contrast to $IO, which contains the name as specified by the USE command.

GT.M does not permit the SET or NEW command to modify $ZIO.

An example where $ZIO contains a value different from $IO is if the environment variable gtm_principal is defined.

Example:

$ gtm_principal="foo"
$ export gtm_principal
GTM>WRITE $IO
foo
GTM>WRITE $ZIO
/dev/pts/8

Notice that $ZIO contains the actual terminal device name while $IO contains the string pointed to by the environment variable gtm_principal.