The system variable named ITERM$ contains the identification
number of the initiating terminal for the current program. This
value is a three-byte string in the form "Tnn", where "nn" is a
value from "00" to "99".
When a program is running in a foreground partition, the value of
ITERM$ is identical to TERM$. However, for a program running in a
background partition, TERM$ will contain a null value and ITERM$
will contain the identification number of the terminal initiating
the background process.