As far as I know, this is what you call a service login hence the installer will ask you what is your "Service User" or something like that. Each user has their own login tied to their roles but the system login is for the server.
I don't use the Global_Logon yet but hopefully the memo or text in the javaBE.ini helps out. I only use the USER_AUTH. You may ask other users if they used that as global. I am speculating it is what it state it is in the text below. This is a special use case where users only have 1 user name and password and only have X amount of allowed connection. So in theory you will have a store that does inventory and all of them uses one userID (ex: "Clerk"). The benefit to this is you only have one user id the trade off is you do not know who did what.
~~~~~~~~from javaBE.ini~~~~~~~~~~
[LOGON_METHOD]
; USER_AUTH if standard UID/Password authentication is used
; USER_AUTH_GLOBAL if pooled connections using single UID/Password is used
; USER_AUTH_GROUP if UID/Password authentication with SAP Message Server
; (load balancing) is used
; USER_AUTH_SSO if SSO2 ticket authentication with SAP Portal Server is used
; USER_AUTH_CUSTOM for a custom login module setup
LOGON_METHOD=USER_AUTH
[GLOBAL_LOGON]
; referenced when LOGON_METHOD=USER_AUTH_GLOBAL
; uses a pool of connections to the SAP backend all utilizing a single
; UID/password
UID=
UPASSWORD=
SHAREDCONNECTION=100
~~~~~~~~~~~end~~~~~~~~~~~~~~~~~~~~