When creating a custom field map, you have the option of enabling backfill on individual maps. Backfill allows you to ignore the source of truth and always update a field.
As a worked example, let's consider this field mapping:
In this example, we are mapping the phone field of Intercom to the phone field of Pipedrive. Intercom is the source of truth (Also known as the source system), meaning the phone value in Intercom will always overwrite the phone value in Pipedrive.
Let's assume that during sync the Pipedrive person has a phone entry but the Intercom contact does not. With backfill enabled, the phone value from Pipedrive will be written to the phone value of Intercom.
Therefore, backfill allows you to ensure that if a value is missing from your source of truth, in this case Intercom, that if the value exists in your target system, in this case Pipedrive, the value will be pushed back from target to truth.
As a further case study and example, if your team updated the phone value in either Intercom or Pipedrive, and Pipedrive is updated before Intercom, enabling backfill would allow you to automatically ensure that Intercom has a phone value if it is added to Pipedrive first.