AT&T 6300 Programmer's Manual page 252

ERR and ERL
Functions
The ERR function returns the error code and
the ERL function returns the number of the
line which contains the error.
Syntax
Remarks
7-96
[ERR I ERL]
When an error handling routine is entered, the
function ERR contains the error code and the
function ERL contains the line number of the
line in which the error was detected.
The ERR and ERL functions are usually used
in IF ... THEN statements to direct program
flow in the error handling routine.
If the statement that caused the error was a
direct mode statement, ERL will contain
65535.
If the line number is not on the right side of
the relational operator, it cannot be
renumbered with RENUM. Because ERL and
ERR are reserved functions, neither may
appear to the left of the equal sign in a LET
(assignment) statement.
GWBASIC error codes are listed in
Appendix A.