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Product quantities are managed in a single tab. The way it works is quite easy: the page presents you with a table of all the combinations for the current product (if there are no combinations, the table simply has a single row). It is up to you to set the initial stock for all the combinations. PrestaShop will use this to determine when a product is soon out-of-stock or unavailable anymore.
Availability preferences
Behavior when out of stock
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