There are mainly 4 types of RFC communication in SAP. They are

1.Synchronous RFC

2.Asynchronous RFC

3.Transactional RFC

4.Queued RFC

Synchronous RFC (sRFC) : In this type of RFC communication,

the calling program waits until the requested processing

step on the remote system has ended and then continues to

work locally.

In other words, both the systems involved must be available

at the time the call is made.

Asynchronous RFC (aRFC) : In this type of RFC communication,

the calling program gives the request to the remote system

and immediately continues to work locally. The requested

processing step is executed on the remote system in isolation.

If the remote system cannot be reached at the time of the

call, the asynchronous calls of the RFC client are lost

Transactional RFC (tRFC) : This type of RFC communication is

similar to asynchronous RFC but by allocating a transaction

id(TID) it guarantees that if a request is sent several

times because of network problems it is processed only

once. Unlike asynchronous RFC, in Transactional RFC the

remote system does not have to be available at the moment

the RFC client program start the call. The data is held in

the source system until the target system is available.

The report program RSARFCSE is called in the background at

regular intervals and tries to place the unsuccessful

requests, identified by their transaction id again.

Queued RFC (qRFC) : This RFC communication is an extension

to the transactional RFC. In this method, all the requests

are queued up(inbound queue and outbound queue) and are

processed in a sequence only if it is certain that all

preceding calls are processed correctly. This method

guarantees that all the requests are processed in the

sequence in which they are received.

This type of RFCs will be used in SCM (APO) systems for CIF

queues etc where requests have to be processed in an order.