This interval is mainly a result of slow conduction through the atrio-ventricular node. Any individual QRS may be represented by only one of these waves. It must be the first deflection of the QRS complex. It must be followed by an R wave. Thus there can be only one Q wave per QRS complex.
All positive deflections are R waves and only R waves.
There can be several R waves per QRS complex. It must follow an R wave. There can be several S waves per QRS complex. There must be no R wave in the QRS complex. Therefore it describes a QRS complex with only one deflection and this deflection is negative.