To find existing sublist line in mapping, the Use as a key field to find existing lines setting can be used.
The Use as a key field to find existing lines setting is not a mapping field, but it’s available in the sublist mapping settings. If set, the setting finds the existing sublist line with “<DestinationRecordField (NetSuite)>” as a key field and updates it. If it doesn’t find the existing sublist line, it will create a new line.
If the setting isn’t used, it will simply create a new line for every item present in the data, irrespective of whether the item was previously present or not.