With source fields you can assign incoming fields to your elements. Depending on the element you need do define multiple source fields as input values.
You can define as many source fields as you want for each element. Depending on the element type and the parser multiple fields will be processed differently. You'll find information on how the parser handles multiple fields in each parser section under source fields. By default the first found value will be processed.
Planed feature: Joining multiple source fields