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Microsoft Dynamics 365 Add-in is a custom business logic that you can integrate with Dynamics 365 (both online and on-premises versions) to modify or augment the standard behavior of the platform.
Microsoft Dynamics 365 Add-ins are some of the most commonly used and powerful approaches to extending the application. The objective of the custom plug-in is to enhance or modify the standard features/behavior of Dynamics 365 by injecting custom business logic into the execution of nearly any task a user performs. For example, Add-ins allow you to expand the user interface, send requests to external resources (like API), CRUD available Entities (add pre and post-logic) and implement another specific business logic for your current tasks.