  An important feature of the EUV spectrum (known as the Big Blue Bump,
hereafter BBB) in Seyfert Galaxies is the narrow range in its cutoff energy E0
from source to source, even though the luminosity changes by 4 orders of
magnitude. Here we show that if the BBB is due to accretion disk emission, then
in order to account for this ``universality'' in the value of E0, the emission
mechanism is probably optically thin bremsstrahlung. In addition, we
demonstrate that the two-phase model with active regions localized on the
surface of the cold disk is consistent with this constraint if the active
regions are very compact and are highly transient, i.e., they evolve faster
than one dynamical time scale.
