The manner in which layers were used in the old Ax was pretty clear, we had
SYS, SYP Used by the core system
GLS, GLP Used by the initial added modules which were bought in ( CRM from Hands Norway, Product builder from Aston (now Tectura) HRM from Circle Capital, Shop Floor from Thy Data Center ).
LOS, LOP Used by some countries to add functionality ( US EDI connector, Germany Cost Accounting from Circon )
DIS, DIP Used by most countries for localisation
BUS,BUP Used initially by partners freely then only by invitation in an early version of IBI (Industry Builder Initiative)
VAR,VAP Used by the partners to put their code in
CUS,CUP Reserved for the customer
USR,USP Reserved for the customers
/Ashlesh