The site taxation policy determines if product and other prices are treated as gross or as net prices in the site. Make sure that you have updated all your prices according to the preferred tax policy before you change this preference.
To upload tax tables from another system, see Tax Table Object Import/Export.
See Taxation Impact On Subscription Fees to understand how tax calculations can significantly impact the Commerce Cloud subscription fees that you pay.
See Line Item Tax Basis if you need details on implementing item-level price adjustments. See Creating a New Site in Business Manager and SiteGenesis and Taxes.
See Understanding VAT Implications, Promotions and Taxation.