The first mapping reads the unload file source and populates comprehensive filter tables.
The second mapping uses the filter tables from the first mapping as lookup tables and populates subset filter tables.
The third mapping reads the unload file source a second time and filters segments using the set of filter tables from the second mapping. PowerExchange resequences the segment data so that it will appear in the same order in the output as it would have appeared in the input file. PowerExchange then creates the output multiple-record unload file. For IMS, this file also includes the necessary header and trailer records.