This paper presents a different approach to classify Self-Mixing (SM) signals operating in moderate feedback regime. A total of 6 distinct classes of SM signals can be defined based on the SM inherent shapes which depend on both the feedback factor C and the linewidth enhancement factor . This classification allows to clearly identify SM signals for which normalization issues can arise and thus for which displacement precision is inherently reduced due to the very nature of signal itself. Finally, it is shown that phase unwrapping approaches can theoretically retrieve displacement with subnanometer precision for usual laser diodes in moderate feedback regime, in the absence of noise, only for values greater than 4.3 approximately.