Thanks Bill!
That info helped me try a couple things but I am still getting errors.
For example, if I left Saleperson ID to Use Sourcefield and didn't fill anything out, I get the error "Must provide 'Source Field' value for 'Use Source Field' rule on the field. It does not matter what I set the If Null to in this instance.
I then tried adding a column and leaving the fields blank, then pointing the Salesperson ID to the field and setting the If Null to Use Defualt. I get the error "Field Salesperson ID does not have a default value."
I then changed the If Null to Use Blank and got the error "Saleperson ID is a required field but value was not supplied."