GammaRegularized[a, z] is the regularized incomplete gamma function .
Mathematical function (see Section A.3.10).
In non-singular cases, .
GammaRegularized[a, , ] is the generalized regularized incomplete gamma function, defined in non-singular cases as Gamma[a, , ]/Gamma[a].
Note that the arguments in GammaRegularized are arranged differently from those in BetaRegularized.
See The Mathematica Book: Section 3.2.10.
See also: InverseGammaRegularized.