Dynamics Ax 2012 Layers Order illustration

The below is an illustration for a better understanding of the Order of Layers in Dynamics Ax. The bottom layer is the core layer and each layer overwrites other from Bottom to Top respectively.

Three layers available only to Microsoft to deliver the base application
Layer	Description
SYS, SYP	System layer: The standard application is developed in this lowest layer. This includes the core application and the localization for most countries.
GLS, GLP	Globalization layer: Includes features developed for some countries that were not yet moved into the SYS layer
FPK, FPP	Feature Pack layer: Includes industry feature packs that are controlled by Microsoft.
Three layers available to partners and ISVs
SLN, SLP	Solution layer: Includes Microsoft endorsed industry solutions.
ISV, ISP	Independent Software Vendor layer: Includes generic or vertical solutions developed by ISV's.
VAR, VAP	Value Added Reseller layer: Includes multi-customer solutions developed by VAR's.
Two layers available to both partners and customers who have a license to access the AX source code
CUS, CUP	Customer layer: Includes customer specific functionality.
USR, USP	User layer: Includes installation specific functionality and customizations

Couple of Benefits of Layers in Dynamics Ax are:

1. Source code Protection
2. Faster and Easier Upgrades 

Other Internal link: Layers in MS Dynamics AX