How to distinguish between the different layers of Microsoft Dynamics AX

Microsoft Dynamics AX Layered Architecture. You can think of the layered architecture of Microsoft Dynamics AX as the layers of an onion. Microsoft Dynamics AX begins with a SYS layer, in which the Core code is written. Nothing and no one can affect the code written in the SYS layer except for Microsoft Dynamics AX development itself. 

Surrounding the SYS layer is the GLS (global), layer. Microsoft development teams have built country specific localizations and other special customizations into these two layers.  Most localizations now exist in the SYS layer, which helps simplify implementations, especially across multi country deployements.  There are also GLS layers for Central and Eastern Europe, Iceland, India, Turkey, China, Brazil and Japan. 

In the BUS (business) and VAR (Value Added Reseller) layers, your chosen ISVs and implementation partners will build customizations that solve the specific needs of your business. The CUS (customer) layer is reserved for your own customizations, while every user at your site can customize their user experience using the USR (user) layer.

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.