Issue of mass upload of Purchase orders thru R4311Z1I

We are planning to upload mass purchase orders as a part of migration of open purchase orders.

We are planning to use R4311z1i.I have a query w.r.t this.

For some orders we have single detail line whereas for some order we have multiple detail lines.

Using application P4311Z1 how do we link the Purchase order Header to the Details in the above scenario where there will be 1 purchase order header  but some PO may have  1 detail line whereas others may have multiple detail lines.

Should the Purchase order in the application be given the Order number or should it get generated fro the Next number set up. If we are supposed to give the Purchase Order no. in the P4311Z1 application in the Header and Details then that can be the link of the Header or details.