Q-API is an interface for buffered data transfer. Data is transferred to the partner system using CPI-C.

This is a set of functions, which places the data temporarily in a database queue, to be processed later by a program running asynchronously.

This SAP interface allows asynchronous data exchange between two systems (R/3, R/2 or an non-SAP system).

As of R/3 Release 3.0 you can use the transactional RFC for buffered data transfer.

The transactional RFC is not supported in R/2.