The SEND buffer is used to accumulate only normal, not expedited, data.

Issuing an APPCCMD CONTROL=SEND macroinstruction to send data does not automatically cause data to flow through the network. Instead, VTAM® stores the data in a buffer in VTAM storage until enough is sent to exceed the maximum SEND RU size that was specified in the session parameters used to establish the conversation's underlying session.

The SEND buffer has nothing to do with logical record size. If the buffer fills up before a logical record is completely stored in it, VTAM sends what is in the buffer and stores the remainder of the logical record in the buffer. VTAM maintains the record's integrity throughout the process.

FMH-5 headers pointed to by the APPCCMD CONTROL=ALLOC and APPCCMD CONTROL=SENDFM5 macroinstructions and FMH-7 headers created by the APPCCMD CONTROL=SEND, QUALIFY=ERROR macroinstruction are sent immediately to the partner transaction program.