Hi all, I found two precisely written post by José A. Estevan   https://blogs.msdn.microsoft.com/axinthefield/embrace-the-extensions-mindset-with-dynamics-365-for-finance-and-operations/   https://blogs.msdn.microsoft.com/axinthefield/embrace-the-extensions-mindset-with-dynamics-365-for-finance-and-operations-2-sysextension-framework/   Also interesting video by Michael Fruergaard Pontoppidan  about chain of command idea under the Extensions.