  The optical depth parameterisation is typically used to study the 21-cm
signals associated with the properties of the neutral hydrogen (HI) gas and the
ionisation morphology during the Epoch of Reionisation (EoR), without solving
the radiative transfer equation. To assess the uncertainties resulting from
this simplification, we conduct explicit radiative transfer calculations using
the cosmological 21-cm radiative transfer (C21LRT) code and examine the
imprints of ionisation structures on the 21-cm spectrum. We consider a globally
averaged reionisation history and implement fully ionised cavities (HII
bubbles) of diameters $d$ ranging from 0.01 Mpc to 10 Mpc at epochs within the
emission and the absorption regimes of the 21-cm global signal. The single-ray
C21LRT calculations show that the shape of the imprinted spectral features are
primarily determined by $d$ and the 21-cm line profile, which is parametrised
by the turbulent velocity of the HI gas. It reveals the spectral features tied
to the transition from ionised to neutral regions that calculations based on
the optical depth parametrisation were unable to capture. We also present
analytical approximations of the calculated spectral features of the HII
bubbles. The multiple-ray calculations show that the apparent shape of a HII
bubble (of $d=5$ Mpc at $z=8$), because of the finite speed of light, differs
depending on whether the bubble's ionisation front is stationary or expanding.
Our study shows the necessity of properly accounting for the effects of
line-continuum interaction, line broadening and cosmological expansion to
correctly predict the EoR 21-cm signals.
