The central concept within OrderCloud.io is (not surprisingly) the Order. This is where Buyers, Sellers, Products, Addresses, and just about everything else come together to form business transactions. A feature that is somewhat unique to this area of the API is that you'll use the same set of endpoints regardless of whether you're a Buyer, Seller, or Supplier. What's important is the order *direction*, which is either incoming or outgoing. Order IDs are guaranteed unique within that context. As a seller, this means you can aggregate lists of incoming orders from many Buyers, or outgoing orders to many Suppliers. You may filter a list by Buyer or Supplier, but it's not required to uniquely identify an Order. (Buyers typically only deal with outgoing Orders; Suppliers with incoming.) Buyers add Line Items to Orders by specifying a Product, Quantity, and (optionally) Spec values. Shipping is applied at either the Order level or Line Item level. Shipments are independent of Orders; you can ship a partial Order, or aggregate items from multiple Orders onto a single Shipment.