The name of approximation method is used in the combination of int & vpa

Int function can solve only simple function, but when a function is very complex, Matlab gives an answer without solving i.e. p = int(f(x),x,0,1). To get the numerical value of the integration I have found, vpa can be used i.e. vpa = (p,3) and it is mentioned in Matlab manual that it approximates p up to three digits. Can anyone explain which kind of approximation (like Simpson, Trapezoidal, Mid-point ...) is used in this algorithm? P.S.- Clearly it is not solved analytically. Thanks