Context : The discovery by Swift and HETE-2 of an afterglow emission associated possibly with short GRBs opened the new problematic of their nature and classification .
This issue has been further enhanced by the observation of GRB060614 and by a new analysis of the BATSE catalog which led to the identification of a new class of GRBs with “ an occasional softer extended emission lasting tenths of seconds after an initial spikelike emission ” .

Aims : We plan a twofold task : a ) to fit this new class of “ hybrid ” sources within our “ canonical GRB ” scenario , where all GRBs are generated by a “ common engine ” ( i.e .
the gravitational collapse to a black hole ) ; b ) to propose GRB970228 as the prototype of the above mentioned class , since it shares the same morphology and observational features .

Methods : We analyze Beppo SAX data on GRB970228 within the “ fireshell ” model and we determine the parameters describing the source and the CircumBurst Medium ( CBM ) needed to reproduce its light curves in the 40 – 700 keV and 2 – 26 keV energy bands .

Results : We find that GRB970228 is a “ canonical GRB ” , like e.g .
GRB050315 , with the main peculiarity of a particularly low average density of the CBM \langle n _ { cbm } \rangle \sim 10 ^ { -3 } particles/cm ^ { 3 } .
We also simulate the light curve corresponding to a rescaled CBM density profile with \langle n _ { cbm } \rangle = 1 particle/cm ^ { 3 } .
From such a comparison it follows that the total time-integrated luminosity is a faithful indicator of the nature of GRBs , contrary to the peak luminosity which is merely a function of the CBM density .

Conclusions : We call attention on discriminating the short GRBs between the “ genuine ” and the “ fake ” ones .
The “ genuine ” ones are intrinsically short , with baryon loading B \la 10 ^ { -5 } , as stated in our original classification .
The “ fake ” ones , characterized by an initial spikelike emission followed by an extended emission lasting tenths of seconds , have a baryon loading 10 ^ { -4 } \la B \leq 10 ^ { -2 } .
They are observed as such only due to an underdense CBM consistent with a galactic halo environment which deflates the afterglow intensity .