The system variable named TERM$ contains the identification
number of the 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
TERM$ is identical to ITERM$. However,
for a program running in a
background partition, TERM$ will contain a null value and ITERM$
will contain the identification number of the terminal initiating
the background process.