The Dynamics 365 for Finance and Operations application layer is primarily represented by application runtime, known as Application Object Server (AOS). AOS runs as an ASP.NET web application hosted on Internet Information Services (IIS). AOS has all the core kernel components (security, metadata, and data access), forms engine, UI interaction service, web services endpoints, and so on. Asynchronous batch processing capability and data management is 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, Enterprise edition application stack is divided into the following distinct models and further divided into multiple packages:

As shown in the image, the following are three models representing the application stack:

• Application platform: This is the lowest-level model 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), task recorder, and mobile framework.
• Application foundation: This contains shared application code used by the different modules in the application, such as number sequence, global address book, source document format, generic report engine, generic tax engine, and so on.
• Application suite: This is the top-level model containing the code of the basic application functionality for each module. Customers, partners, and ISV can extend the application suite to add additional functionality to fulfill their unique business requirements. Application suite started as a monolithic big model containing the entire application area; however, probably over time, this will get divided into multiple models to create independent applications out of Dynamics 365 for Finance and Operations.

Splitting the application code stack into multiple models provides many benefits, including modular code, faster compile time, and better servicing models. More details about the models and packages are available in Chapter 9, Building Customization in this book.