tablename_update is used to REWRITE a record from the file. The parameters passed to the stored procedure are the values of all the columns in the row, in the order of the columns in the database. The timestamp column and identity column (if present in the table) are not passed to the stored procedure.
For example, based on the Sample XFD, you might want to create the following stored procedure for rewriting a record:
create procedure ftestdat_update @ft2_key char(4), @ft2_key1_seg1 char(2), @ft2_key1_seg2 char(2), @ft2_altkey2 char(4), @ft2_number char(9), @ft2_info char(10) as update ftestdat set ftest2_key = @ft2_key, ftest2_key1_seg1 = @ft2_key1_seg1, ftest2_key1_seg2 = @ft2_key1_seg2, ftest2_altkey2 = @ft2_altkey2, ftest2_number = @ft2_number, ftest2_info = @ft2_info where ftest2_key = @ft2_key grant execute on ftestdat_update to public