Dynamics Ax 2009 Layers

Dynamics AX 2009 consists of sixteen application object layers that contain all the
elements you see in the AOT.
These layers can be looked at as an onion with multiple layers. In the middle is the
core application in the SYS layer and the outermost layer is the user layer USR.
Therefore, when any application element is being executed the system will look at
the outermost code layer first to see if there is any code for that element; if not, it peels a layer off the onion, and tries the next layer. When it hits a layer where the element exists, it will use the code from this layer, and will not continue to peel off layers to find code for that element in the innermost layers. 

Layers with their description

SYS The standard application is implemented at the lowest level, 
the SYS layer.The application objects in the standard 
application can never be deleted.

GLS Country/region specific changes will be developed in GLS 
Layer.For e.g as you all know that Tax structure differs 
from country to country.So such localization functionality 
can be developed in GLS layer.

HFX HFX is an application object patch layer reserved by 
Microsoft for future patching or other updates.

SL1, SL2,or SL3 A layer where the distributor can implement 
vertical partner solutions. 

BUS When a business partner creates their own generic solution, 
their modifications are saved in the BUS layer and the top-
level application objects are used.

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. 
Such modifications are saved in the VAR layer.

CUS The supervisor or administrator of an end user installation 
might want to make modifications that are generic to the 
company. Such modifications are saved in the CUS (CUStomer) 
layer.

USR End users might want to make their own modifications, such 
as in their reports.These modifications are saved in the USR 
layer.