It sounds a little silly, right? Either it's in stock or it isn't? Well, just like everything else on Google Shopping, it's a little more complex.
What is [availability]?
Availability is an attribute within the Merchant Centre, used to tell users and Google whether your product is in stock, out of stock, available for pre-order, or currently in backorder.
If you're unsure how the latter three differ, you're not alone. We'll use this short guide to describe and explain how to differentiate between these values.
[in stock]
If you're currently accepting orders for this product and can fulfil the purchase request, choose [in stock]. It's important that you only choose this value if you can guarantee you can deliver the product in a timely manner, and to all of the locations you support in your product data and account delivery settings.
[out of stock]
This attribute should be used when the relevant product is not available for purchase and you do not know when or if it will be back in stock.
[preorder]
This value should be used for products which haven't yet been released but customers are still able to purchase. In this instance, you must provide the [availability_date] to indicate the day that the product will become available for delivery.
[backorder]
Products marked as [backorder] aren't currently available. However, if you're still accepting orders and can dispatch the item as soon as it becomes available again, you should select this value.
