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.