It says to use only one list.
Or you make for every category the product list in PHP and output that everytime to the template.
Or you make a combined sorted list with category and product data in it and output that to the template which then checks changes in the category data to insert a category header.
In general, I think the first method is the more efficient.

send me a pm with the username you registered at dev.xoops so I can add you.