How does the product type filtering work?

Shopify allows assigning a product type to each product. This can be done on the Shopify product editing page:

You can limit the random recommendations displayed to products of the same type of the current product by enabling the following option in the Frequently Bought Together preferences page, in the Recommendation tuning section:

Let's say, for example, that you're seeing a product page whose product type is 'Shirts'. The random recommendations displayed will only be products with product type 'Shirts'.

Please note that, if there are not enough products matching the selected criteria, the recommendations will be automatically completed using random recommendations.