we have g_errbuf := SQLERRM in the exception blocks . What is assigned to variable g_errbuf here?

Question

  EXCEPTION
    WHEN OTHERS THEN
      g_errbuf  := SQLERRM;
      g_retcode := 2;
      write_log('Y',
                to_char(SYSDATE, 'HH24:MI:SS') || ' - ' ||
                'Data Error happened having Batch ID = ' || p_batch_id ||
                ' and invoice_ulid = ' || l_process_data_rec_type.invoice_ulid);
      write_log('Y',
                to_char(SYSDATE, 'HH24:MI:SS') || ' - ' || 'Error ' ||
                l_procedure_name || ' : ' || g_errbuf);
      IF c_get_ap%ISOPEN
      THEN
        CLOSE c_get_ap;
      END IF;

Answer 1

No better place to start the official documentation. If the exception is thrown, SQLERRM will contain the error message.

Example:

set serveroutput on size 999999
DECLARE
  l_num NUMBER;
BEGIN
  l_num := 'x';
EXCEPTION WHEN OTHERS THEN
  dbms_output.put_line(SQLERRM);
END;
/
ORA-06502: PL/SQL: numeric or value error: character to number conversion error

So in your code, the variable g_errbuf will contain the error message of the statement that is erroring out.