This feature allows to make sure that required quantities of products that customer is about to purchase are actually available in Erply. This feature works by intercepting order creation where it requests current stock levels for products in cart and if some products are no longer available in desired quantity or completely out of stock, then customer is redirected to Cart page where such products are highlighted.
You can enable/disable this feature by changing a setting “Verify Product Stock in Checkout” on this page in Magento Admin.