Systems Network Architecture transmits normal data through the network in a chain of request units. Before these units are transmitted, the partners agree to the maximum size of each of the elements. Because there is some fixed amount of work to handle each unit, regardless of its size, network efficiency can be improved if each unit contains the maximum amount of data.

When a SEND macroinstruction completes, the amount of data that remains in the SEND buffer is less than or equal to a maximum unit size. A subsequent SEND macroinstruction from the local application will repeat this operation.

Certain macroinstructions cause all of the data in the SEND buffer to be transmitted. This can be either under the explicit control of the application by the use of a flush type of SEND macroinstruction or it may be implicit in that the flush is automatically performed by VTAM®. For example, VTAM automatically flushes the SEND buffer, for half-duplex conversations, as part of the operation for the transition of the local transaction program from SEND to RECEIVE state. This automatic flushing ensures that it is in SEND state; it also has all the data that was transmitted by the local transaction program.