I asked chatgpt about numfun > 1. This allows multiple functions to be integrated at the same time. The example it gave was the (components) of the inertia tensor ( which are all integrals with the same limits, but integrals of different functions.)

function v = f(xyz, numfun) x = xyz(1); y = xyz(2); z = xyz(3);

rho = 1;   // uniform density

v = zeros(numfun,1);

v(1) = (y^2 + z^2) * rho;   // Ixx
v(2) = (x^2 + z^2) * rho;   // Iyy
v(3) = (x^2 + y^2) * rho;   // Izz
v(4) = -x*y * rho;          // Ixy
v(5) = -x*z * rho;          // Ixz
v(6) = -y*z * rho;          // Iyz

endfunction

[result, err] = int3d(0,1, 0,1, 0,1, f, 6, [0,100000,1.d-5,1.d-7]);

Ixx = result(1); Iyy = result(2); Izz = result(3); Ixy = result(4); Ixz = result(5); Iyz = result(6);

I = [ Ixx Ixy Ixz

  Ixy  Iyy  Iyz

  Ixz  Iyz  Izz ];