When you buy a French Chablis, it will be made only from Chardonnay grapes grown in a small district in France, because that’s required under French law.
Likewise, when you buy a Sancerre, it can only be a sauvignon blanc from the eastern Loire Valley. A Beaujolais can only be made from the Gamay Noir à Jus Blanc grape grown in a district of Burgundy.