Hi 
The recurring option is for sale subscription.If this field is enabled we use this product for subscriptionsRefer the following docs

https://www.cybrosys.com/blog/how-to-create-a-new-subscription-product-in-odoo-16-subscription
https://www.cybrosys.com/blog/subscription-management-module-in-odoo-16

Hope it helps

For normal products the “Can be Sold" check box is marked and then that product is available for ordinary sales order.

If we want to use a product as subscription-based product and want to generate recurrent invoices, then "Recurring" checkbox needs to be enabled as well as “Can be Sold”.

Once it's done from Subscription -> Products, we can view all the products recurring products. If we clear the filter "Can be Recurring", all the products (both normal sellable products and recurring products) will be visible from Subscription app.

Now, if we want to create a sales order through Subscription app or Sales app and want to sell both ordinary products and recurring products all we have to do the following steps:

1. In the order lines, we will be able to search through & select all the products available in the inventory that are sellable. All products means both ordinary sellable products and recurring products. And it’s possible to add both of these products in the order line.

2. In order to confirm the sales order, we have to configure a Recurrence option in the Sale order form. There is a field named “Recurrence”, and in that field the recurrence method whether the subscription type is monthly or yearly or weekly or quarterly needs to be specified.

3. After specifying the recurrence method, we will be able to confirm the sales order. But if we don’t follow through that, the sales order can’t be confirmed.