The Rain system has a robust ordering module, and is flexible to accommodate your preferences. It may be that you have an ordering system you currently use that you don't want to move away from. If so, we have you covered: you can continue to order through that system, and just do Receiving Orders to add the inventory to the Rain system.
Our Purchase Order system allows you to work with Vendors you've saved in the system to put together a printable order you can send to the vendor. You may see 'PO' as a short way to write Purchase Order and 'RO' as short for Receiving Order, in our documentation, and the ordering system is often referred to as PO/RO.
When you create a Purchase Order, think of it as your original reference set of the items on order. Once you've sent it, you're basically waiting to start receiving the items. They may come all in one shipment, or they may be scattered out in several shipments, but you can always go back to the Purchase Order and see that original reference set of all the items (that's what you see when you first open it). You can also see any Receiving Orders that resulted from those shipments, and each of those will be sub-sets of that original reference set of items.
Receiving Orders are created either from a Sent Purchase Order or from scratch, if you are not using our Purchase Orders. Every shipment you receive will generate a Receiving Order. The easiest way to access them is if they are tied to a PO, you can go to Purchasing and see any RO's that have been created so far from the original Purchase Order (as pictured), and/or you can continue receiving on the order. The other method is just to start with a Receiving Order.