The pre-order liquid variable is useful when you need to run Shopify theme code specifically for products on pre-order

PreProduct adds a Shopify metafield whenever a product has a pre-order listing created. Then removes it when the listing is finished. Namely: product.metafields.preproduct.preorder

To add logic/elements to products on pre-order, you can use the following: 

{% if product.metafields.preproduct.preorder %} 
     This sentence only loads when a product is on pre-order.
 {% endif %}

….and the opposite is possible too. To only add logic/elements when a product isn’t on pre-order, you can add the following:

{% unless product.metafields.preproduct.preorder %} 
     This sentence only loads when a product is not on pre-order.
 {% endunless %}