How does quantity limit settings work in Pre-order Helper app?

Quantity limits can be set at two levels: product and variant. 
The variant-level quantity limit takes precedence over the product-level limit. If no limit is set, customers can order as many products as they wish. Once the limit is reached, a "sold-out" message will be displayed on the product page.

To determine if the quantity limit has been reached, the system will check the variant stock. This calculation will change if the "Allow customers to select shipping date?" checkbox is checked. In that case, the system will store the number of items sold in the database and check whether that number has reached the limit.