If you like to keep a reserve of stock for some products, you can set a Minimum Stock level against them.
If the stock level falls below this level, the required quantity will appear in the Products Required work stack, and when adding products to a purchase order.
Setting a minimum stock level against a product
You can change From Settings - Inventory - <Category> - Products.
You can either do this from the item record, via the Stock tab, or directly from the Products Grid.
The Stock Required values will take this value into account, along with any stock on hand, stock on order, and reservations.
Visibility in the Products Required work stack
The Products Required work stack in the Stock module shows products that require stock to be ordered.
The Required values include the amount needed to bring the stock level up to the minimum, as well as unfulfilled reservations.
Visibility when adding items to a purchase order
When adding items to a purchase order, the Min REQ column will reflect the minimum stock value for that item. The REQ column value will take this value into account.