For this example, the diameter of the taper is very large, so it uses a Gaussian beam. Instead of the common scalar Gaussian, it uses vectorial beam, similar to the case when the scalar gaussian beam passes an microscope object lens. Thus it has NA, the Numerical Aperture. As for other parameters,such as lens diameter, beam diameter, I would rather not change the default, since they may change the effective NA.
NA is n*sin(theta) where "n" the refractive index of the material where the beam source is located, and theta is the geometrical angle which allows the the ray enters the lens.
Numerical aperture - Wikipedia
Please note that "n" is not "neff" which is the mode property.
As for 'uniaxial anisotropic PML', it is a legacy PML. Please refer
Usually, if you do not know how to modify, simply use the default settings. You only need to modify when the default settings do not work, or violate physics.