receiver determination step in the BPM and multiline container

i have made a file2file scenario where:

there is a BPM,

the BPM has a send step which contains a receive step,

a transformation that mapps the incomin message to two messages(Split-messgae case),

and then a FORK step which sends the two messges to through 2 SEND steps to the intended receiver services.

It didnt work......and i followed a blog-> /people/sudharshan.aravamudan/blog/2005/12/01/illustration-of-multi-mapping-and-message-split-using-bpm-in-sap-exchange-infrastructure ............... which suggested to have a

"receiver-determination" step in the BPM before the SEND step.... it says that this

"receiver-determination" step will put the message in a MULTILINE container..

i m confused bout this step ........ can anybody explain.....

please help