Distribution of the stellar spectral types in the XSL according to the SIMBAD database. Unspecified stars are grouped in the “∼” class. The dark gray color represents the full library of 628 stars. Set 1 (orange) refers to the 39 stars selected by PPXF out of the full library to construct an optimal template 1. Set 2 (blue) refers to 32 stars selected from a random half of the full library and set 3 (emerald) refers to 33 stars selected from the other half. Sets 2 and 3 have 15 and 17 stars, respectively, in common with Set 1. The alternating light gray and white vertical regions divide the spectral classes for easier visualization.