  We explore the influence of optical Fe II quasi-continuum on the measured
spectral parameters in the 4150-5500 A range for the spectra of Type 1 active
galactic nuclei (AGNs). We assume that the broad line region is composed of two
sub-regions: the very broad line region (VBLR) and the intermediate line region
(ILR). We constructed a large set of synthetic AGN spectra by taking different
portions of the VBLR and ILR contributions, where initially the VBLR and ILR
model spectra were constructed on the basis of prototypes of two observed
spectra with dominant VBLR (i.e. ILR) emission. To investigate the influence of
the optical Fe II quasi-continuum on the AGN measured spectral parameters, we
fit the power-law continuum and emission lines in a set of model spectra, as
commonly done for observed AGN spectra. We then compared the spectral
parameters obtained after the fitting procedure with those of the model. We
find that the optical Fe II quasi-continuum can be very strong in the case of
spectra with strong and very broad Fe II lines and it is difficult to fully
separate it from the power-law continuum. This gives the effect of a slightly
underestimated H$\beta$ width and underestimated fluxes of the H$\beta$ and Fe
II lines, while the continuum flux is then slightly overestimated. The most
affected spectral parameters are the line equivalent widths (EWs), especially
EW Fe II, which may be strongly underestimated. We discuss the possible
underlying physics in the quasar main sequence, as implied by the results of
our spectral modelling. We find that the set of AGN model spectra assuming
different ILR and VBLR contributions can aptly reproduce the quasar main
sequence, that is, the full width at half maximum (FWHM) H$\beta$ versus Fe
II/H$\beta$ anti-correlation, where both parameters in this anti-correlation
are strongly dependent on the ILR and VBLR contribution rate.
