To return extended error information.
EXTERR returns "extended" error information provided by versions of DOS 3.0 and greater. For versions of DOS earlier than 3.0, EXTERR always returns zero. The single integer argument must be in the range 0-3 as follows:
Value of n
0 : Extended error code
1 : Extended error class
2 : Extended error suggested action
3 : Extended error locus
The values returned are not defined by GW-BASIC, but by DOS. Refer to the MS-DOS Programmer's Reference (version 3.0 or later) for a description of the values returned by the DOS extended error function.
The extended error code is actually retrieved and saved by GW-BASIC each time appropriate DOS functions are performed. Thus when an EXTERR function call is made, these saved values are returned.