How do I know when stock runs low?

Open your sublocation from Locations, then select Configure Products. In the product editor, open Inventory and enable Track Inventory to reveal Low Stock Threshold.

If you use Stock Groups, each group has its own Low Stock Threshold field. Use Add Stock Group to add a group when setting up the product.

The catalogue's Customize product columns dialog includes a Low stock badge display option.

For tracked products without stock groups, the catalogue shows Low stock when stock is above zero and at or below Low Stock Threshold. At zero, it shows Sold out. For example, with a threshold of 5, stock of 5 or 4 shows Low stock, while stock of 10 shows no stock-warning badge.