I wish to know more about the boundry type - interface. I use it to specify the face between the region before the rotor and the volume between the rotor blades.

Thank you.

I think you might be confused with interface and internal (or interior). Internal is used to separate different fluid zones and mesh control. Interface, however, allows you to assemble different mesh components. Let's say you are modeling a flow going through two pipes. One of them has smaller diameter than the other. Let's say the flow comes in at the small pipe and goes out at the large one. Fluent allows you to mesh those two pipes separately. And then the outlet of the small pipe and the inlet of the large pipe will have type of interface. In Fluent, you go to Define->Grid Interface to match them. The common part to those two interfaces will have fluid going through, while the uncommon part will be walls.