Will use the value of the selected field unless that value is empty in which case it will use the value of another field to add to the output.

This example adds a field Validated sale name that has the value from the Sale title field, unless that is empty. If it is empty it will have the value from field Name, which in this example is the company name.
Configuration
Field – Select the field from the source that is checked for an empty value.
Fallback field – Select the field from the source that is used if the value in Field is empty.
Field name – The name of the newly added field.
Notes – Optional description to describe the reason this transformation is added. Convenient for later reference.