The semiotic description of music structure aims at representing the high-level organization of music pieces in a concise, generic and reproducible way as a low-rate stream of arbitrary symbols from a limited alphabet, which results into a sequence of " semiotic units ". In this context, the purpose of the System & Contrast model is to address the internal organization of the semiotic units. In this report, the System & Contrast model is approached from different angles in relation to varied disciplines : cognitive psychology, music analysis and information theory. After establishing a number of links between the System & Contrast model and other approaches of music structure, the model is illustrated on studio-based popular music pieces, as well as on music from the classical Viennese period.