Layers in Microsoft Dynamics AX 2009

Layers are a hierarchy of levels in the application source code that enable you to make modifications and additions without interfering with the application objects in the next lower level. When you make an object modification on one level, the modification overshadows the object on a lower level. You could, for example, decide to add e-mail information to a standard form. The addition would be saved on our level only. The revised form replaces the standard form. However, you can always return to the original form at the next lower level by removing the new form.
Three customer groups have an interest in adding and modifying application objects:

• Application developers who create the standard application.
• Business partners.
• End users of Microsoft Dynamics AX.

Here are descriptions of the application object layers in Microsoft Dynamics AX:
USR - User - Individual companies or companies within an enterprise can use this layer to make customizations unique to customer installations.
CUS - Customer - Companies and business partners can modify their installations and add the generic company-specific modifications to this layer.
VAR - Value-Added Reseller - Business partners use this layer, which has no business restrictions, to add any development done for their customers.
BUS - Business solution - Business partners develop and distribute vertical and horizontal solutions to other partners and customers.
SL1-SL3 - Certified solutions - Partners certified under the Microsoft Dynamics Industry Solution (MDIS) program distribute their solutions in the SL layers.
HFX - Hotfix - The Dynamics AX team delivers critical hotfixes using the HFX layer.
GLS - Global solution - The Dynamics AX Global Development and Localization team provides a set of GLS layers that contain country-specific functionality for regions in which Dynamics AX is released.
SYS - System - This is the lowest model element layer and the location of the standard Dynamics AX application. Only Microsoft has access to the element definitions at this layer.
Objects defined at higher levels of the stack override objects defined at lower levels of the stack. That means USR Layer Object will cast a shadow on the same object in, for example, the VAR Layer. Objects are stored in a separate file (*.aod) on each layer whenever they are saved from the MorphX development environment client. Element definitions are read from these files and dynamically composed by the Dynamics AX runtime. Object instances are created on either the server or the client based on the model element definition.