I am following Chapter 9 of the Using Drupal book (2nd Edition). This chapter introduces Drupal Commerce.

I have got to the stage where I have added my first Product Display (on page 373).

In the Content Type settings, I noticed that, for some reason we left all options unchecked for the the field called "product types that can be referenced". This confused me because, when creating my first product display all those unchecked product types actually *where* offered as potential references.

So, please could someone explain what the "product types that can be referenced" field does on the Product Display content type?