In the mSUGRA (minimal supergravity) breaking scenario, the stop particle typically appears at energies reachable at the LHC. Other sfermions, notably the partners of up, down, strange and charm are assumed to be degenerate in mass, and also heavier than the stop. Something similar holds for the stau.
Why is the third generation different in mSUGRA (not degenerate as the first two), and why is the mass hierarchy inverted wrt. the Standard Model sector (3rd generation sparticles lighter)?
(I guess these features are not neccessarily specific to mSUGRA, but might apply to more general models as well.)