Very important to installation of two types of filters respectively Primary type which is obvious in the picture The other type is shown in the pictures down. Main air filters are rarely installed on individual room return (or supply) air registers, but identifying the presence and the location of both supply and return air registers will help us understand whether or not the system uses central returns or individual room air returns, or a mix of both. That in turn will tell us .whether or not to look for filters at central return registers