Nousha,
it is a question of standards. The structure of an IDoc is defined by an IDoc type. Among other things, the IDoc type contains information about which data is saved where (line and offset), The IDoc type is assigned to the message type. Whereas the message type contains only the semantics of the message (for example, material master data), the IDoc type contains the exact structure of the document for a message type. This means there can be different versions of a material master IDoc, for example, each with a different IDoc type. This enables versioning, which means that IDoc scenarios can be developed further in a compatible way.
Check out the preview of this edi book from sap press which has a scenario of an inbound edi X12 850 PO to ORDERS IDoc interface: https://www.thali.ch/files/Shop/Documents/013603_Leseprobe.pdf
Hope that solves your issue.