You can create a one-to-one relationship between the records and associate them using Lookup Field. For example, you can link the Customer module with the Contact module by applying a lookup field with the Contact module. The lookup field allows users to select the Customer which is associated with this contact. For example, John Doe who works for Acme Co., Ltd. 

Note: a one-to-one relationship between the Customer and the Contact represents that one contact can only have one customer.

Follow the steps below to create a lookup field for the Contact module and link it to the Customer module.

1. Navigate to the Contact Module detail page, then click Create Field. You will be directed to the Create Field page.

2. Enter “Company” in Label field.
3. Select Lookup for data type, then Lookup Module, Lookup Criteria & Display and Display Column options will appear.
4. Lookup Module is a target module that you want to associate with. In this case, select Customer as the Lookup Module.

5. For Lookup Criteria & Display, you can set the criteria of associated records that users can search and select the display columns when users use this lookup field. 
6. Click on icon, you will be directed to the Create Lookup Criteria & Display page.

7. In Search Criteria section, select fields that users frequently use to search the Customer records, e.g., Customer Name and Customer ID. For the condition, we suggest you to always select Contains and leave Value field blank.

8. For Display Columns and Sort Order, you can select fields that you want to show on search results and sort them in ascending or descending order.

9. Click Save. You will be directed to Create Field page.
10. For Display Column, you can choose one of the display fields that you saved from Lookup Criteria & Display steps.

 11. Set field attribute, e.g., Is Unique, Allow Navigate and Allow Search.

 12. Click Save.

 13. Then place the Lookup Field on Field Layout (both Edit and Detail view). 

 14. Click Add Record, you should see lookup fields that linked to the Customer module available for your selection. 

 15. Click on icon to search for the customer with specified criteria in step 7, then select the desired company by clicking on the record or on  icon.

 16. Click Save, this record will be linked to the customer.