Dual-write allows the real-time integration of data between D365 CRM and F&O environments.
Well! While integrating two systems it is must that both the system knows how to find the respective record in other system’s table. This is where the Integration key comes into the picture.
Integration Key tells Dual-write engine, what columns to be taken into consideration while finding the respective record in D365 CRM.
Let me explain this with an example,
Consider in F&O there is a Customer (it synchronizes to the Account table in D365 CRM) with the Customer Number as DEMF001 and this has already been synchronized to D365 CRM. Now, next time if any update is made to the DEMF001 record in F&O the updated details should synchronize back to the right record. How will Dual-write engine know which is the respective record in D365 CRM? It is with the help of Integration Key. For this scenario I would configure the integration key as below.

Note: Company is equivalent to F&O’s Legal Entities.

Let us understand, how this is related to Alternate Keys in D365 CRM. In the D365 CRM, the main purpose of Alternate Key is to facilitate an integration with another system that usually do not use GUIDs as the primary key and have a different Primary Key. Also, Alternate Key in CRM makes sure that the value for the column(s) which is/are part of the Alternate Key is unique or else it will throw an error. It is best practice to define the Alternate Key and it will automatically be translated to as the Integration Key in Dual-Write.
Note: If your table has multiple Alternate Keys make sure to manually check the Integration Key whether the right one is applied or not.
Migrate Integration Key from one environment to another
The question whether the Integration Key can be transferred from one environment to another is quite a valid question, so to answer it yes, it is possible. However, it is not a straightforward process. For the Integration Key to be available at the destination environment, along with moving over the Dual-write mappings, you need to make sure that the table that you are trying to move has the Alternate Key defined with the right columns. Alternate Keys will then be translated as the Integration Key.
Conclusion:
This way even the Integration Key would be automatically added.