Network tiers in MSD Axapta

AX 2009 is built as a three-tier solution. This means that it has a data-tier where the data is stored, a logic-tier where all business logic is executed, and a presentation tier that presents information to the user and receives input from the user.
As an AX developer you can select to have the piece of code you are writing be run on the client or the server. The client will represent the presentation-tier and the server will represent the logic-tier. The AX server is also called the Application Object Server or AOS. The data-tier consists of either an MS SQL or an Oracle database.
There are also two completely different clients available in AX. The regular Windows client, also known as the rich client, is the one most users think of when we talk about the AX client, but there is also a business connector client.
The business connector client can be used by a third-party application to integrate to AX, and it is also used by the enterprise portal.
In addition to these three layers we also often talk about a fourth component in the tier model, the file-server. The file-server is not really a tier in itself as it does not execute any code but only stores the code.