Implement a plugin for Author or a Server Application Programming Server application when you want to automate library tasks. The context in which you want your code to run and how you want to automated the tasks it performs determines which is best for your scenario. If you want to automate the actions based on an event or user action in Author then you should implement a plugin for Author. If you want to automate actions based on events or user action in an application or system that is external to Author-it then you should implement a Server Application Programming Server application.