  We present a spectral study of the ultraluminous Be/X-ray transient pulsar
Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER)
observations during the system's 2017--2018 giant outburst. The 1.2--10~keV
energy spectrum of the source can be approximated with an absorbed cut-off
power law model. We detect strong, luminosity-dependent emission lines in the
6--7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington
regime, is seen to evolve into a broad Fe-line profile in the super-Eddington
regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex.
An asymmetric broad line profile, peaking at 6.67 keV, is possibly due to
Doppler effects and gravitational redshift. The 1.2--79 keV broadband spectrum
from NuSTAR and NICER observations at the outburst peak is well described by an
absorbed cut-off power law plus multiple Gaussian lines and a blackbody
component. Physical reflection models are also tested to probe the broad iron
line feature. Depending on the mass accretion rate, we found emission sites
that are evolving from ~5000 km to a range closer to the surface of the neutron
star. Our findings are discussed in the framework of the accretion disk and its
implication on the magnetic field, the presence of optically thick accretion
curtain in the magnetosphere, jet emission, and the massive, ultra-fast outflow
expected at super-Eddington accretion rates. We do not detect any signatures of
a cyclotron absorption line in the NICER or NuSTAR data.
