  Superfield constraints were often used in the past, in particular to describe
the Akulov-Volkov action of the goldstino by a superfield formulation with
$L=(\Phi^\dagger \Phi)_D + [(f\Phi)_F + h.c.]$ endowed with the nilpotent
constraint $\Phi^2=0$ for the goldstino superfield ($\Phi$). Inspired by this,
such constraint is often used to define the goldstino superfield even in the
presence of additional superfields, for example in models of "nilpotent
inflation". In this review we show that the nilpotent property is not valid in
general, under the assumption of a microscopic (ultraviolet) description of the
theory with linear supermultiplets. Sometimes only weaker versions of the
nilpotent relation are true such as $\Phi^3=0$ or $\Phi^4=0$ ($\Phi^2\not=0$)
in the infrared (far below the UV scale) under the further requirement of
decoupling all additional scalars (coupling to sgoldstino), something not
always possible (e.g. if light scalars exist). In such cases the weaker
nilpotent property is not specific to the goldstino superfield anymore. We
review the restrictions for the Kahler curvature tensor and superpotential $W$
under which $\Phi^2=0$ remains true in infrared, assuming linear
supermultiplets in the microscopic description. One can reverse the arguments
to demand that the nilpotent condition, initially an infrared property, be
extended even in the presence of additional superfields, but this may question
the nature of supersymmetry breaking or the existence of a perturbative
ultraviolet completion with linear supermultiplets.
