  Supergiant fast X-ray transients (SFXTs) are characterized by a remarkable
variability in the X-ray domain, widely ascribed to the accretion from a clumpy
stellar wind. In this paper we performed a systematic and homogeneous analysis
of sufficiently bright X-ray flares from the SFXTs observed with XMM-Newton to
probe spectral variations on timescales as short as a few hundred of seconds.
Our ultimate goal is to investigate if SFXT flares and outbursts are triggered
by the presence of clumps and eventually reveal whether strongly or mildly
dense clumps are required. For all sources, we employ a technique developed by
our group, making use of an adaptive rebinned hardness ratio to optimally
select the time intervals for the spectral extraction. A total of twelve
observations performed in the direction of five SFXTs are reported. We show
that both strongly and mildly dense clumps can trigger these events. In the
former case, the local absorption column density may increase by a factor of
>>3, while in the latter case, the increase is only by a factor of 2-3 (or
lower). Overall, there seems to be no obvious correlation between the dynamic
ranges in the X-ray fluxes and absorption column densities in SFXTs, with an
indication that lower densities are recorded at the highest fluxes. This can be
explained by the presence of accretion inhibition mechanism(s). We propose a
classification of the flares/outbursts from these sources to drive future
observational investigations. We suggest that the difference between the
classes of flares/outbursts is related to the fact that the mechanism(s)
inhibiting accretion can be overcome more easily in some sources compared to
others. We also investigate the possibility that different stellar wind
structures, rather than clumps, could provide the means to temporarily overcome
the inhibition of accretion in SFXTs.
