po import

we want to import the po's from third party systems in apps.they will send the po headers,lines,distribution details in a single file.we have lot of po's.that pos having multiple lines and distributions.we have to prepare the format file and have to send to client,based on our format they will send the data.here my question is how we have to identifie this is header information ,line and distribution and that po having how many lines and how many distributions.based on which, we have to identifie the header lines and destributions.

pls share the your ideas.