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What is difference configurable products and simple product with the custom option?

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A configurable product and a simple product with custom option look almost similar on the front end. The main difference is that in a configurable product inventory can be easily managed as each variant of a product is a separate simple product with its own inventory. But in a simple product with the custom option the inventory of a custom option variant cannot be managed.

The biggest difference is:

The configurable product creates many simple products which are combined from the options Simple product with custom options doesn’t create the new products, it’s simply the additional options along with the product order.

For more info, you can refer to this link: https://bsscommerce.com/blog/magento-2-simple-product-with-custom-option-vs-configurable-product/