  An open question in SN Ia research is where the boundary lies between
'normal' Type Ia supernovae (SNe Ia) that are used in cosmological measurements
and those that sit off the Phillips relation. We present the spectroscopic
modelling of one such '86G-like' transitional SN Ia, SN 2021rhu, that has
recently been employed as a local Hubble Constant calibrator using a tip of the
red-giant branch measurement. We detail its modelling from -12 d until maximum
brightness using the radiative-transfer spectral-synthesis code tardis. We base
our modelling on literature delayed-detonation and deflagration models of
Chandrasekhar mass white dwarfs, as well as the double-detonation models of
sub-Chandrasekhar mass white dwarfs. We present a new method for 'projecting'
abundance profiles to different density profiles for ease of computation. Due
to the small velocity extent and low outer densities of the W7 profile, we find
it inadequate to reproduce the evolution of SN 2021rhu as it fails to match the
high-velocity calcium components. The host extinction of SN 2021rhu is
uncertain but we use modelling with and without an extinction correction to set
lower and upper limits on the abundances of individual species. Comparing these
limits to literature models we conclude that the spectral evolution of SN
2021rhu is also incompatible with double-detonation scenarios, lying more in
line with those resulting from the delayed detonation mechanism (although there
are some discrepancies, in particular a larger titanium abundance in SN 2021rhu
compared to the literature). This suggests that SN 2021rhu is likely a lower
luminosity, and hence lower temperature, version of a normal SN Ia.
