Periodic Boundary Settings in RF Module

Hi,

I do not know how I should do setting for k vector of Floquet periodicity.
What is KF in user defined menu tab of "k vector for Floquet periodicity"?
Does it show the direction of periodicity?
Let us say that the unit cell of a subwavelength structure is periodic in x and y directions and the incident beam propagates in z direction and normal incident angle.
What will be the settings for KF?

I appreciate if somebody answers this.

Thanks,
Mohammad