This option causes the ASB LOOKUP statement (and corresponding BASICplus function) to always return a positive value. Otherwise, in order to more closely mimic AMOS, it will return a negative value if the file is an exact multiple of 512 bytes (since this makes it most likely that the file was originally random or contiguous, and thus you would have returned a negative LOOKUP value under AMOS.) Note that while this is a nice trick, it is not infallible, since the chances are 1 out of 512 that a sequential file will also be an exact multiple of 512 bytes. We strongly recommend that you review the use of LOOKUP in your programs and convert your logic to use ABS to convert negative values to positive, rather than simply negating the value.
