I think that now I understand a little bit more about your requirement.
So what are you trying to model is a process to gather Purchase Requests or Purchase Orders?
It doesn't sounds like just one process, it sounds as a bunch of processes and instances gatehering information about a big operation to me.
If you create a business process, you need to clearly identify the goal of that process and build your tasks around that goal.
For example if the process is called: New Purchase Order Process
The process will contain all the tasks required to create a new Purchase Orders, these tasks can be enter in all the purchase requests associated to that order.
The other thing that you need to know is that a same task can be used to store different snapshots of the work, for example if you say that
One Purchase request have multiple parts, you can just have one task which allows you to enter each of these parts in different times. When you have entered all
the required or necessary parts you can complete the task and move the process forward.
Hope it helps..