Layers

http://www.codeproject.com/Articles/313659/Tips-and-Enhancements-in-Dynamics-AX  ---> ax 2012 modifications over 2009

Layers

The following table describes the application object layers in Microsoft Dynamics AX:

Layer	Description
USR	The user layer is for user modifications, such as reports.
CUS	The customer layer is for modifications that are specific to a company.
VAR	Value Added Resellers (VAR) can make modifications or new developments to the VAR layer as specified by the customers or as a strategy of creating an industry specific solution.
ISV	When an Independent Software Vendor (ISV) creates their own solution, their modifications are saved in the ISV layer.
SLN	The solution layer is used by distributors to implement vertical partner solutions.
FPK	The FPK layer is an application object patch layer reserved by Microsoft for future patching or other updates. For more information, see Patch Layers.
GLS	When the application is modified to match country or region specific legal demands, these modifications are saved in the GLS layer.
SYS	The standard application is implemented at the lowest level, the SYS layer. The application objects in the standard application can never be deleted.

Each layer has a corresponding patch layer that can be used to incorporate updates to your application or to store conflicts when you import models into a layer. For more information about patch layers