Typically, you should use transmission for surfaces separating different rooms and transparency for surfaces inside the same room. 

Transmission in ODEON works differently than in real life. The sound energy ‘hitting’ a transmission surface in ODEON is not lost inside the wall. This is because transmission and absorption in ODEON work independently of each other. Therefore the volumes on either side of a transmission wall should be isolated from each other. by design ODEON keeps 90% of the emitted rays in the source room; and only 10% of the rays are left transmitted to the other side. This means that, when performing transmission calculations, you should always use at least the number of rays specified by the Precision setting in the Room Setup, to ensure that a sufficient number of rays are transmitted through the wall.

On the other hand, for surfaces inside the same room, e.g. curtains, plants, part of the sound energy is supposed to travel through them and part of it is supposed to be absorbed or reflected back. For such surfaces, Transparency should be used. A value of eg. 0.3 would mean that the surface allows 30% of the sound to travel through, without any modification, while the rest 70% is subject to absorption (or reflection), as it happens with any other normal surface. Transparency can be directly translated to the number of ‘gaps’ on a surface.  If blinds are partly closed and leave app. 40% gaps, then transparency is simply 0.4. For curtains, a typical empirical value for Transparency is 0.5. Learn more about transmission in this video tutorial.