After creating a Lookup field, the modules with the Lookup field are also linked to a target module (Lookup Module). In this example, the Contact module is now linked to the Customer module, but on the other hand the Customer module is not yet linked to the Contact module. In order to complete the link between the two modules, you will need to Add Relation for the Customer module as well. 

After creating a Lookup field, follow the steps below to add Contact relation to Customer module.

1. Navigate to Customer module, then click on Relationship tab.
2. Click Add Relation.

3. Enter the relation name.
4. Select relation type and related module. If you select Bind relationship and select the Customer module, you will see Lookup Field that has been created previously available for selection.
5. Select display field (this option will show only available fields that you add in create lookup field steps).

6. Click Save.

Note: If relation has not been added, the linked records will not be available in the lookup module (target module). In this case, if you add relationship table in Field Layout, you will only see the new records that have been created after the modules are linked but not the ones that have been created prior even if there is the Lookup field.