  (Abridged) Classification schemes for YSOs are based on evaluating the degree
of dissipation of the surrounding envelope, whose main effects are the
extinction of the optical radiation from the central YSO and re-emission in the
far-infrared. Since extinction is a property of column density along the line
of sight, the presence of a protoplanetary disk may lead to a misclassification
when the system is viewed edge-on.
  We performed radiative transfer calculations, using the axysimmetric 3D
radiative transfer codes RADMC and RADICAL, to show the effects of different
geometries on the main indicators of YSO evolutionary stage, like the slope of
the flux between 2 and 24mum, the bolometric temperature and the optical depth
of silicates and ices.
  We show that for systems viewed at intermediate angles the 'classical'
indicators of evolution accurately trace the envelope column density, and they
all agree with each other. On the other hand, edge-on system are misclassified
for inclinations larger than ~65deg. In particular, silicate emission, typical
of pre-main sequence stars with disks, turns into absorption when the disk
column density reaches 1e22cm-2, corresponding e.g. to a 5e-3 Msun flaring disk
viewed at 64deg. A similar effect is noticed in all the other classification
indicators studied alpha, Tbol, and the H2O and CO2 ices absorption strengths.
This misclassification has a big impact on the nature of the flat-spectrum
sources (alpha ~0), whose number can be explained by simple geometrical
arguments without invoking evolution. A reliable classification scheme using a
minimal number of observations is constituted by observations of the mm-flux
with both a single dish and an interferometer.
