In each of the complexes, there are two types of apartments: wellness and non-wellness. Wellness is for everyone under 21 and non-wellness is 21 and older. If you are 21+, you can choose to live in a wellness apartment (for example if you want to room with friends under 21), but you waive all rights to bring alcohol into the apartment. Even if you are 21 and you are caught with alcohol in a wellness apartment, you can be termed on the spot. Generally, it's safer to choose non-wellness if you're 21+, just in case.
There's nothing different about the apartments; it's just a classification of whether alcohol is allowed.