Auxiliary passive elements shall be understood to include such elements as attenuators@ transformers@ filters and equalizers@ applied as separate units to be combined with other separate sound system units to constitute a complete sound system. Notes 1. ?? Auxiliary passive elements which are intended to be mounted as components in a sound system unit and to form a non-interchangeable part of such a unit@ are not within the scope of this Recommendation. Some characteristics are to be specified for all types of auxiliary passive elements. These are given in Chapter I and Section One. Further characteristics to be specified for the different types of auxiliary passive elements are given in Section Two@ Clauses 17@ 18 and 19. 2. ?? Auxiliary elements may sometimes include electronic amplification@ in which case these elements are considered as amplifiers and are dealt with in IEC Publication 268-3@ Sound System Equipment@ Part 3: Sound System Amplifiers. 3. ?? If a network has more than four terminals as e.g. a cross-over network@ it is nevertheless treated in this Recommendation as an assembly of four terminal networks.