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1. Since many Data fields are repeatable, it is required that the fields that are mapped to them be placed inside of a field group. Furthermore, field groups should contain fields that are mapped to Marc subfields of the SAME Marc Data field. For example, a field group named 'Description' contains fields 'Extent' and 'Dimensions', if 'Extent' is mapping to '300$a', then 'Dimensions' must be mapped (if there's a mapping at all) to a subfield in the 300 data field, such as '300$c'.
2. Mappings in all descendants of a field group must reference the same data field. For For example, a field group named 'Description' contains a field 'Extent' and a field group 'Dimensions' with a field called 'Dimension', if 'Extent' is mapping to '300$a', Dimension must be mapped to a subfield in the 300 data field.
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