RFC and programming information for RFC client/server programs can be summarized as follows:
An RFC connection is always initiated by an RFC client program.
An RFC connection is always built up in two steps:
Connection from an RFC client to the Gateway
Connection from the Gateway to an RFC server
There are different ways of starting or connecting to an RFC server program:
An RFC server program can be started by the Gateway, by the currently running SAPGUI or by the currently running application server.
You can register an RFC server program at an Gateway. The program waits for RFC requests from SAP systems.