Sales Documents that are optional : Sales Quote, Sales Order, Shipment, Credit Memo.
Sales Documents that have an effect on the General Ledger : Sales Invoice, Shipment, Cash Receipt, Credit Memo.
Sales Documents that have an effect on Inventory : Shipment (or Sales Invoice), Credit Memo.
Dates are very important to pay attention to when creating these documents. Make sure that the Sales Order date is before the Shipment Date, for example.