Many options exist for inventory tracking. See Admin > Configuration > Stock for details.
Common configurations are:
1. Don’t track stock
Check stock level = false Subtract stock = false Allow Checkout = true
This configuration is well suited for stores that sell virtual products, which can’t be depleted.
2. Track stock, but permit backorders
Check stock level = true Subtract stock = true Allow Checkout = true
This configuration is ideal for stores that sell products where manufacturers can fill re-orders easily and out-of-stock or discontinuation situations are unlikely.
3. Track stock, do not permit backorders
Check stock level = true Subtract stock = true Allow Checkout = false
This configuration would be a good match for stores that sell unique or limited quantity products, such as artisan produced goods.
Status handling for Subtract stock = true
Stores that track stock (i.e. have Subtract stock = true) also need to decide on whether changes are required to the status of a product when the quantity in stock goes to zero.
The configuration setting
Products status in Catalog when out of stock should be set to controls this. When set to 0, a product will change status to Disabled when the last one is sold. When set to 1, no status change will occur.
What about Variant Stock?
At the current time, Zen Cart does not track the stock of individual product variants (a t-shirt’s stock of large versus medium, for example). There are commercial modifications that address this requirement however, such as the Products’ Options’ Stock Manager (POSM).