If I understand your question correctly. You are reading documentation that talks about creating a SZ order in JDE from EDI flat files.
I don't know anything about F47131, but we do use the F47011/12 and R47002C to create Sales Orders in our system from flat files.
Let me provide a few comments that may help.
1. You do not have to use SZ as the document type when doing EDI. We, for example, create SO sales orders through EDI. SO is also the document type we use for most phone and fax orders. It all depends on how your company wants to do things which is far more than we can discuss on this post.
2. The document type that gets created is usually defined on processing options for the P4210 version you choose for EDI. I hope this doesn't get confusing. The Sales Order gets created by job R47011 (which gets called by R47002C), which has processing option for a Sales Order Entry version. This means you need to choose/create a version of P4210 for your EDI orders. On that version, there is a processing option called Order Type which will determine what document type the EDI order will be.
I hope this helps. I would recommend you find a consultant or another JDE company near you that knows EDI. There are many decisions to make that can get quite confusing.
Jer