Selling and buying can each run at two depths, chosen per company (and changed later, with one guard described below). Depth is about how many document stages a transaction passes through.
[Standard] selling runs the full chain: sales order → waybill → sales invoice → receipt. The order records the promise, the waybill records the delivery, the invoice records the charge, the receipt records the money. [Simple] selling skips the order and waybill stages: you raise the sales invoice directly, and the receipt follows it. Buying mirrors this exactly. [Standard] purchasing runs purchase order → goods receipt → purchase invoice → payment; [Simple] purchasing starts at the purchase invoice.
Choose Standard when the stages are genuinely different moments or different people's work — when orders are approved before goods move, when deliveries are partial, when you need to know what is on order and awaited, not just what has been billed. Choose Simple when the sale or purchase is effectively one event — over-the-counter sales, small firms where the same person orders, receives, and pays — and the extra stages would be ceremony rather than control.
In Simple depth the skipped documents do not merely hide: their menus disappear and the documents cannot be created, so nobody can accidentally run two depths at once. Documents from a previous Standard period remain viewable if you switch.
One guard: you can move from Simple to Standard at any time, but moving from Standard to Simple is refused while open orders or draft delivery/receipt documents exist — the application names what must be finished or cleared first, so a promise in flight is never stranded without its paperwork.