What are the layers in Axapta and what is the purpose of each layer?

8 layers:
SYS- Lowest layer where the standard application is implemented.
GLS-MBS certifies and distributes a solution that has not been developed in-house.
DIS-Country specific legal demands are saved in this layer
LOS-Distributor implements local partner specific solutions
BUS-Partner generic solutions, here Axapta ensures that the top level application objects are used
VAR-Industry specific solutions are saved in this layer
CUS-Company generic modifications done by the Administrator
USR-End user may wish to save his/her own modifications, for eg: repor