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Herish SurendranHerish Surendran 

When to use APIs , outbound message , inbound message , platform events , external services. Which criteria lets us decide which one to use and when.

AnudeepAnudeep (Salesforce Developers) 
Hi Herish, 

It depends on your business use case

When it comes to APIs, there are a number of different APIs that developers can use to integrate with a Salesforce organization. But sometimes it’s difficult to know what each API is for and when to use it. I recommend reviewing this article from Salesforce Developer Blog to learn more

Outbound messaging on the other hand uses the notifications() call to send SOAP messages over HTTP(S) to a designated endpoint when triggered by a workflow rule. With platform events, the goal is to reduce the number of point-to-point integrations and expand on the existing capabilities of other integration options such as Outbound Messaging, Apex Callouts, and the Streaming API.

Coming to external services, I recommend reviewing the following example scenarios to understand when to use them 
  • Make your new Salesforce org users automatic collaborators for external, org-related applications. Maybe you want users to have access to a payroll information app so that they can look up their own timesheet and pay data. You can register an external service and create a flow with triggers that takes inputs such as user ID. When you create a user, the triggers fire and add the user as a collaborator who has access to the payroll app.
  • Connect to a credit service that determines if credit is extended to an account in your org. You can register your external service and create a flow that includes inputs like order amount and credit terms. When the flow runs, it updates the credit terms for the order associated with the account.
To learn more about external services, see this Trailhead

If you find this information helpful, please mark this answer as Best. It may help others in the community. Thank You!

Anudeep