In a database, if one record is related to another record, it has a field that points to a field in the related record. This is called either a "lookup field" or a "relationship attribute."
For the field in one record type to point to a field in another, the record types must have defined entity relationships Metadata configurations that allow for related records to have lookup fields, which let users link the related records to other records. Any related records can appear in a list that is visible from the record that they have been related to..
For an N:N (Many to Many) relationship, you cannot import the lookup field.
For an N:1 (Many to One) relationship, you can import the lookup field.
To import related data, valid data in the source file column must establish the relationship.
All data in the column must point to the same column in the same or another source file or to one field.
Source lookup mapping defines the relationship using two columns of source files.
System lookup mapping defines the relationship using a column in a source file and a field of an existing record type.
Each value in the column must reference a record in the directed column or field. The Import Data Wizard can look up the data either in records that are already in Microsoft Dynamics CRM, or records that are in another source file that is part of the same import.
Only one value in a source column imports data to a lookup field. For example, if you are importing data from a "Recipients" column in your source data to the To field for e-mail messages, there can be only one address in the Recipients column.
When you select the field in the related record type, select a field that has unique data in each record. If the referenced values are not unique, the record will not import.
Suggested Preparation in Your Source Files
Verify that data in your source files lookup fields points to valid records. To identify lookup fields, see Entity Reference (Resource Center).
If records in a source file contain multiple values in a field that contains relationship data, edit the data so it contains one value.
Optional Changes in Microsoft Dynamics CRM
If you are creating custom lookup fields, create a relationship A definition of how records may be related to each other. Relationships defined between entities control how Microsoft Dynamics CRM presents options for records to be linked to each other in the application. between the two record types. The Import Data Wizard creates this relationship. It always creates a many-to-one relationship between the record type you are importing that has a lookup field in it and the referenced record type. If you need to change the type of of relationship between these record types, after you finish importing data, use the Customization area to change the relationship type.
Notes
If a case record references a contract record with a Contract End Date(expireson) or Billing End Date(billingenddate) field value earlier than the current date, the case record will not import. To work around this, change the value in Contract End Date and Billing End Date during the import, and after completing the data import, change the values in Microsoft Dynamics CRM.