Reorder suggestions top stock back up to your Max level. When the count on hand is below zero, you choose how that negative is read: as a counting gap to ignore, or as units you owe a customer.
How a count below zero is sized
A count below zero counts as zero when Stockroom sizes an order. With a Max level of 2 and -1 on hand, the suggestion is 2, not 3. That is deliberate: a count below zero is nearly always a counting gap - an oversell against shelf stock, a receipt nobody recorded - rather than units you owe a customer, so filling it would size the order to repair your books instead of your shelf. Your sales history already counts every unit that sold, including the ones that took the count negative, so real demand is in the suggestion either way. The same floor applies to transfer suggestions and to Fill suggested quantities in the purchase order editor. It is the default rather than a rule - see below if a negative count means something different in your store.
The negative still shows on the row, so you can always type a higher quantity if you genuinely owe those units. If the count is simply wrong, correcting it is the better fix - see Stock adjustments: corrections with a paper trail or Count your inventory with stocktakes. Once the count sits at zero or above, suggestions fill the whole gap back up to your max level.
If a negative count means units you owe
Some stores order to a customer rather than hold stock, so a count below zero is a real backlog: sell one machine you do not stock and you are one unit short until the supplier ships. If that describes your store, go to Settings > Restocking and turn on Count negative stock when sizing orders.
With it on, the count below zero is added to the order instead of being treated as zero. A Max level of 2 with -1 on hand then suggests 3 - enough to clear the unit you owe and refill to your max. It applies everywhere an order is sized: the Reorder list, automatic restock drafts, Fill suggested quantities in the purchase order editor, and transfer suggestions. A transfer is still limited to what the other location can spare above its own max level, so turning this on will not empty a healthy location.
The setting is off to begin with, and it is one choice for the whole store, not per product. Leave it off if your negatives are usually counting gaps - filling those sizes the order to repair your books rather than your shelf, and a badly wrong count can ask for a very large order.
Order back to zero when there's no suggestion
Counting negative stock only helps an item that has a Max level or a suggestion to fill up to. An item you only ever order for a customer often has neither - no reorder point, and too few recent sales for a suggestion - so it never shows up, even at -2.
For those, turn on Order back to zero when there's no suggestion, just under Count negative stock when sizing orders in Settings > Restocking. It can only be ticked while that setting is on. An item with no reorder point and no recent sales that falls below zero is then suggested for just enough to bring it back to zero: -2 on hand orders 2. The Reorder list marks those rows Back to zero.
Nothing is saved on the product. Set a level, or let new sales build a suggestion, and that takes over on its own. An item sitting at exactly zero is never suggested this way - otherwise every sold-out item in your catalog would be. Stock already on order counts against the shortfall, so it is not ordered twice, and automatic restock drafts and transfer suggestions follow the same setting.
📸 SCREENSHOT: Reorder list with a row showing the Back to zero badge
Good to know
Both settings are one choice for the whole store, not per product.
Items excluded from reorder, and items that don't track inventory, are never suggested - negative or not.
Where each level comes from is in Set reorder points and stock levels.