As the document described, the room effect is dominated by two effects: early reflection and late reverberation. Both are influenced by the room size, room shape, and the material of walls of the room. If the room is able to be defined properly, the options of material of walls and the parameters for adjusting the output of early reflection and late reverberation should be provided. They are independent factors but all would influence the room effect.
As described in the VIVE 3DSP document, the reflection level sets the volume level of the reflection. It is applied on all resulting signals of the reflection effect; The reflection rate is the parameter of sound reflection ability of the wall. It is applied on only the specific reflection of the specific wall.
Hope that this explanation answers your questions. If not, feel free to ask any questions you have.
Prof. 3D Sound