Floquet Modes CST MS Explain

Can someone explain what the floquet modes are? In addtion, when I set a waveguide port at a certain boundary I have the option of specifiying a certain amount of modes. What is the difference between these modes a floquet modes?

waveguide use when u want to simulate normal incident
floquet used for general any oblique angle
floquet mode only operate with unit cell as it simulate ur design as periodic structure