After you import products, the convertproducts.asp script reads through the products table and synchronizes the links between categories and products in the prodcategories table.
To overcome time limitations on very large databases, you can specify ranges of product catalogids, and convert chunks of the database at a time.
Convertproducts.asp will then use this to update the prodcategories table and link that product to the categories listed in the level3 field.
Note: If youare using the level3 field for something else, you can change the field being used by editing convertproducts.asp and changing the following line to have a different field name.
const subcatfield="level3"