  More than 3000 spectroscopically confirmed Type Ia supernovae (SNe Ia) are
presented in the Zwicky Transient Facility SN Ia Data Release 2 (ZTF DR2). In
this paper, we detail the spectral properties of 482 SNe Ia near maximum light,
up to a redshift limit of $z$ $\leq$ 0.06. We measure the velocities and
pseudo-equivalent widths (pEW) of key spectral features (Si II $\lambda$5972
and Si II $\lambda$6355) and investigate the relation between the properties of
the spectral features and the photometric properties from the SALT2 light-curve
parameters as a function of spectroscopic sub-class. We discuss the
non-negligible impact of host galaxy contamination on SN Ia spectral
classifications, as well as investigate the accuracy of spectral template
matching of the ZTF DR2 sample. We define a new subclass of underluminous SNe
Ia (`04gs-like') that lie spectroscopically between normal SNe Ia and
transitional 86G-like SNe Ia (stronger Si II $\lambda$5972 than normal SNe Ia
but significantly weaker Ti II features than `86G-like' SNe). We model these
`04gs-like' SN Ia spectra using the radiative-transfer spectral synthesis code
tardis and show that cooler temperatures alone are unable to explain their
spectra; some changes in elemental abundances are also required. However, the
broad continuity in spectral properties seen from bright (`91T-like') to faint
normal SN Ia, including the transitional and 91bg-like SNe Ia, suggests that
variations within a single explosion model may be able to explain their
behaviour.
