The Dynamics 365 for Finance and Operations application layer is primarily represented by application runtime, known as AOS. AOS runs as an ASP.NET web application hosted on IIS. AOS has all the core kernel components (security, metadata, and data access), forms engine, user interface interaction service, web services endpoints, and so on. Asynchronous batch processing capability and data management are provided by a Windows service running on each application server. The following diagram shows the key components of the server and how they are stacked together:

The Dynamics 365 for Finance and Operations application stack is divided into multiple packages and models. Splitting the application code stack into multiple packages provides many benefits, such as modular code, and better servicing models, as shown in the following diagram:

As shown in the preceding diagram, the following are three key packages representing the application stack:

• Application Platform: This is the lowest-level package and contains application code, handling the core application functionalities, such as runtime and data access, workflow, batch framework, SSRS reporting, Data Import/Export Framework (DIXF), business events, data feeds, task recorder, and mobile framework.
• Application Foundation: This contains a shared application code used by the different modules in the application, such as number sequence, global address book, source document format, generic report engine (GRE), and generic tax engine (GTE).
• Application Suite: This is the top-level package containing the code of the basic application functionality for each module. Customers, partners, and independent software vendors (ISVs) can extend the application suite to add additional functionality, to fulfill their unique business requirements. The Application Suite started as a monolithic big package containing the entire application area; however, over time, this got divided into multiple packages to modularize code and create independent applications out of Dynamics 365 for Finance and Operations.