Just for my own remembering (and anyone who wants to know it), I post a table which summarizes all the development layers within Dynamics AX 4.0. It is easy for me to check my own blog. ;-)
Layer
Descrition
ID range
USR
User
Individual companies or companies within an Enterprise can use this layer to make customizations unique to customer installations.
50001-60000
CUS
Customer
Companies and business partners can modify their installations and add the generic company-specific modifications to this layer.
The layer is included to support the need for in-house development without jeopardizing modifications made by the business partner.
40001-50000
VAR
Value-added reseller
Business partners use this layer, which has no business restrictions, to add any development done for their customers.
30001-40000
BUS
Business solution
Business partners develop and distribute vertical and horizontal solutions to other partners and customers. Solutions in this layer can be protected with license keys with a signed agreement for the Dynamics AX Add-On Solutions program.
20001-30000
LOS
Local Solution
The Dynamics AX solutions offices certify and distribute strategic local solutions developed in-house.
18001-20000
DIS
Distributor
The Dynamics AX Sustained Engineering team delivers critical hotfixes using the DIS layer
160001-18000
GLS
Global solution
The Dynamics AX Global Development and Localization team provides a set of GLS layers that contain country-specific functionality for regions in which Dynamics AX is released
8001-16000
SYS
System
This is the lowest model element layer and the location of the standard Dynamics AX application. Only Microsoft has access to the element definitions at this layer.
1-8000