AT&T 6300 Programmer's Manual page 256

ERROR
Statement
7-100
If the value of the numeric expressions is
greater than any error codes used by
GWBASIC, then the ERROR statement will
generate a user-defined error. This user-
defined error code may then be handled in the
error trapping routine (see the ON ERROR
statement in this chapter).
Note: To define your own error, use a value
that is greater than any used by GWBASIC
error codes. (It is preferable to use the highest
available values, so compatibility may be
maintained if more error codes are added to
GWBASIC.)
If an. error statement specifies a code for
which no error message has been defined,
then GWBASIC responds with the message:
Unprintable error.