IBM 3708 Planning And Installation page 167

Alternate Control Sequence Character
If the terminal uses a control sequence that introduces characters other than, orin
addition to, an ASCII ESC (X'JB'), this 1.byte field contains the alternate
character. The TeleVideo® 925 and 950, for example, use the SOH (01) character as
a control sequence introducer in addition to ESC.
Intermediate Control Sequence Character
Some terminals generate an intermediate control sequence character when sending an
ESC sequence to the 3708. This field defines the intermediate control sequence
character to the 3708. The 3708 discards and ignores this character when the
character is received after the ESC (or after the alternate control sequence
character). For example, the IBM 3161 sends ESC!
nn
for certain keys. The!
(X '21') is defined as the intermediate control sequence character for these terminals.
If the terminal is an ANSI X3.64 terminal, there is no need to define the left bracket
as an intermediate control sequence character. The intermediate control sequence
character cannot be the NULL (X'OO') character.
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3708 Planning and Installation