where to find a catalog field for EDI (ORDERS)

Hello,

I'm looking for a kind of segment/field catalogue to be able to determine and be sure at 100% where a value of an IDOC (ex. ORDERS <=> VA01) is located in a sales order...

(In tcode WE30, it's by example the object ORDERS05 used to create the sales order)

I suppose the company who are doing the mapping between SAP and other format (EDIFACT, XML, ...) must have this ... but is it possible to have or find somewhere this kind of documentation?

thanks for your help!

Pascal.