We need to assume that clothing manufacturing has an impact on the environment, therefore it's almost impossible to say that a specific brand is sustainable, as it would mean that they have 0 impact on the environment. For example a brand could claim to being carbon neutral, which means that they don't contribute to the increase of CO2 in the air in the manufacturing process. However, a brand that claims to be carbon neutral should also include the office activities, the offices themselves, the marketing activities, travel and make sure that everything they do is 0 impact or carbon neutral.