Extra brut champagnes receive little or no dosage, from 6 g/l to zero. Extra brut are usually presented as such, or they can be rosé, blanc de blancs or blanc de noirs that in addition are extra brut.
Extra brut champagnes are meant to better express the terroir, given the reduced amount of added sugar. Some houses may label their extra brut champagnes as brut because they feel it may scare away less savvy consumers who may assume they are harsh champagnes. For example, all Bruno Paillard champagnes are extra brut, but they do not market them as such. They eventually released their zero dosage champagne: Dosage : Zéro, which is spelled on the label.
One extra brut champagne that I particularly appreciated during a tasting session is Barons de Rothschild Extra Brut with its ridiculous yet significant dosage of 1.5 g/l.