This is compatible only with OpenCart 3.x.x.x version. Type 2 is basically to enable Notify me Button / Notify Form for out-of-stock products. The add to cart button stays the same. Therefore customers can choose to purchase the product (Mostly Pre-Order Scenario) or set an alert to the product.

The major difference between the Type 1 and Type 2 are

1. OCMOD files are updated
2. catalog/view/theme/default/template/extension/module/oc3/notifyform.twig updated