How can I configure the value of "currentlyNotInStock" field of a product variant ?

[tiepbui]

When I use Storefront API to get a variant from my development store -->  there is a field named currentlyNotInStock and I see that its value is always false regardless of its settings in the store backend. While as you can see in your official API documentation --> The description for this boolean field is: "Whether a product is out of stock but still available for purchase (used for backorders)." 

So can anyone help me to explain what this field means more clearly and how/where I can configure its value in my store backend ?

Many thanks.

[culverizer]

currentlyNotInStock evaluates to true when the inventory option "Continue selling when out of stock" is checked on a product variant and the quantity available is <= 0.