Just after disgorgement a “liqueur de dosage” – a blend of – most times- cane sugar and wine (sugar amounts up to 750 g/litre)- is added to adjust the levels of sugar in the Champagne and other bubbles when bottled for sale, and hence the sweetness of the finished wine. Today sweetness is generally not looked for per se, dosage is used to fine tune the perception of acidity in the wine. Wines labeled Brut Zero, more common among smaller producers, have no added sugar and will usually be very dry, with less than 3 grams of residual sugar per litre in the finished wine. The following terms are used to describe the sweetness of the bottled wine:

Extra Brut (less than 6 grams of sugar per litre)
Brut (less than 12 grams)
Extra Dry (between 12 and 17 grams)
Sec (between 17 and 32 grams)
Demi-sec (between 32 and 50 grams)
Doux (50 grams)

Source material: Wikipedia https://en.wikipedia.org/wiki/Champagne