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Using the Magic ID lookup relationships, Dataimporter can automatically perform a VLOOKUP on your behalf, to link related records from the source org, to the newly created records in the Target Org.
Under the Account lookup you will see that the mapping has been completed automatically for you, with the AccountId from the source org being mapped to the Account Lookup via MagicId in the Target Org.
What Dataimporter will do in the background, is take the source and target Ids of the parent records that were created, and perform a match, so that when the Lookup values are populated in the target org, they are the same records which existed in the source org.
MagicId can only be used to link objects which are included in the Migration Template, and the referenced object must be run before the current object.
If you try to include an object as a MagicId which is either not included in the Template, or is set to run after the current Job you will receive an error.