Fields in LSMW

Hi Experts,

I am learning LSMW and I have I question to ask.

While using Direct method , BAPI or IDOCs method how developer will know which fields are necessary in "Maintain Source Fields".

As when using Recording method the fields which are compulsory/mandatory are directly  mentioned in the recording. But how will I know which are the fields that I need to mention in 3rd step of LSMw while using Direct , BAPI or IDOC method.

Thanks.