Flat files in Data transfer

Iam able to do tutorials for Direct input and batch input method.I tried doing BAPI and IDOCs too.The point where Iam getting strucked is with flat file.

For example If i want to transfer Purcharse order or vendor master information,I need to put valid data in text file.Lets say for material master, the valide material type is 'XYZ',if i put '123' in flat file,it throws the error that 123 is not valid material type(during the session processing).So I need to put only valid data in the flat text file.How to go about this?

Hope you understood my question.

Another question is that does ALE deals with creating segments.In my understanding

LSMW is a tool which helps in data transfer with following ways

i)Direct input method

ii)Batch input method - recording

iii)BAPI - using built in business objects

iv)Idcos - using inbout idoc processing where we create ports.

BDC is another tool like LWMW,where the ways are call transaction and session.

Is what I understood correct?Any changes?