Relationships between local surface brightness, d_DLR, and Si IIλ6355 pEW for SN Ia sub-types. Left: Local surface brightness compared with the d_DLR of volume limited ZTF DR2 sample. The fit shown with a black solid line on the plot is used to estimate a d_DLR value (vertical dashed line) based on the chosen surface brightness magnitude of 20.1 mag (horizontal dotted line). Middle: Si IIλ6355 pEW plotted against velocity for three different sub-types of SNe Ia: 91T-like (represented by squares), 99aa-like (represented by diamonds), and normal (represented by circles). The data points for each SN sub-type are shown with increasing levels of host galaxy light contamination (25, 50, 75, and 100 per cent) from top to bottom. The different colours represent different galaxy types used to simulate the contamination effects. This allows for the comparison of how various galaxy types influence the measured Si IIλ6355 pEW for different SN sub-types and how the pEW is affected by increasing galaxy contamination. Right: Local surface brightness plotted against the pEW of Si IIλ6355 feature. The grey dotted line represents the equivalent value of d_DLR where we applied our cut of d_DLR = 0.2, based on the corresponding surface brightness magnitude of 20.1 mag. The red and blue dashed line represents the regression line fitted to all the data and the data with the d_DLR cut, respectively. The red and blue shaded regions represent the confidence interval around the corresponding regression lines. A strong correlation between the pEW of Si IIλ6355 and local surface brightness and its effect on sub-typing is shown.