The product category table is a linking table, it has a unique id for each entry and stores the product id and the corresponding category id. This table provides the information about what products belong to what categories.
The PowerStore is designed to be an 'out of box' solution that works already. The db structure was engineered the way that you see it to fit the needs and requirements of the PowerStore Solution Pack. The mass importation of products and their corresponding data into the PowerStore db is not a supported feature so you will not find any documentation about it. The items in the store are meant to be added through the stores admin area.
