Re: Exercises on developing X-Cart 5 - task 3
This module simply displays the ISBN filed and its value if an attribute with name ISBN exists and if it has a value. It shows within specific "list" - in this case product details page under description.
This exercise module was most likely coded when the Specifications tab did not exists or did not show attributes or something like that. With or without the module since you created the ISBN attribute it will show on the Specifications tab - the cart loops through all attributes and shows them on this tab by default.
The whole point of the exercise is to teach you how you can show some info on the product page and how you can loop through product attributes.