Hi Manuel!
The Group/Server Selection is working like follows:
After specifying the System ID and Message Server SAPGui will first check for an entry sapms<System ID> in \windows\system32\drivers\etc\services.
Then it will connect to the Message Server on the host you provided in the field "Message Server" and will collect the system configuration. If the connect was successfull it will present all Logon Groups and all Servers belonging to the system.
If you now select a server, your connection will always be opened to this server.
If you select a Logon Group, the Load-Balancing mechanism for that logon group will dynamically select the current logon server via message server for the connect.
Logon Groups are always listed before the servers.
There is one default Logon Group with the installed system: SPACE.
Logon Groups are maintained in transaction SMLG.
The active server within a Logon Group can be shown in SMLG by Goto -> Load Distribution
Logon Group SPACE has no name in SMLG (the name is blank --> SPACE)
(Logon) Group based server selection during login is the only way to ensure that a server is reached in HA configurations when connecting with GUI or RFC-Clients to a HA configured system.
If you select a server in above example, the message server connection is only used to get a list of currently available servers. Your entry in SAPGui is static and will not change if the server is not available. In this case you will not get a connection to the system.
regards
Peter