There are three layers available only to Microsoft to deliver the base application.
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.
There are 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.
There are 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