How to run RFWT0020 to update withholding tax data

Can anyone explain the difference between these two options when running RFWT0020 to update withholding tax data? Data Regeneration vs. Changes to Source Tax Code? I can't find any mention of the two options in the program documentation 😔

When I do test runs with the two different options, the program is selecting difference documents to update. If we are just turning on the 1099 functionality now (beginning with the 2011 tax year), should I run the program once with Regeneration mode and once with Changes to Source Tax Code???

Thanks for any advice!

Allison