Most common is the SAP-Gui connect over the ServiceNr of the Installation,
also HTTP or HTTPS is required for some applications,
in rare cases it my be needed that an OS or DB connect like telnet or ssh is to be opened.
In general the sapserv3 for Europe, other Nr's in US, Singapure, or India,...,
are connecting the customers saprouter or VPN or ISDN entry point,
from where the Support people are going either to SolMan at customer site,
or to an WTS where the SAPGUI with customer config
and other tools are present to use in your environment.
and this connection need to setup, and opened for dedicated timeframe,
between some min and 99days are possible for one time system and service,
this can be done from SolMan or in SAPNET with the required authorisation.
https://service.sap.com/sap/support/notes/79411 - AS/400: 5250 connection to AS/400 customers