A Field map is mapping between the fields of a target object and a source object. When creating a new target record from a source record, if values of some source fields need to be copied to related target fields, field maps are used. Mapping is possible only between compatible field types such as Date to Date, Lookup to Lookup (lookup to the same object), Checkbox to Checkbox, and so on.

A target field can be mapped to up to 3 source fields. This is called Additional Mapping. If the first source field value is blank, target field is set to the value of second source field if it is non-blank, or to the value of third source field.

A target field in a field map can also be mapped to a constant value instead of to a source field.