Results from PCA. Top: linear combinations of the first two PCs determined from PCA applied to the spectra extending to +70 d and between 6000–7000 Å. Bottom: a linear combination of Amplitude₁ and Amplitude₂ plotted versus its orthogonal (30°) rotation. In addition to the CSP-I SE SN sample, PCA resutls are shown for the normal Type Ia SN 2013gy (Holmbo et al. 2019) and unpublished CSP-II spectra of the Type Ic superluminous SN 2015bn (Phillips et al. 2019). Solid curved lines highlight subtypes of He I-rich SE SNe (see Sect. 5.3). GMM clusters are indicated with one and two sigma contours for each cluster using dashed and dot-dashed lines (see Sect. 5.3.1 and Table 3), respectively. To guide the eye, curved arrows are over-plotted highlighting the temporal evolution of SNe IIb and SNe Ib.