From what I gather, ReframeRT is a way of interpreting the source footage and playing it back at a specified rate. The most common example would be to have 60fps source footage in a 30fps project (settings -> format -> frame rate) and setting it to play back at 1/2 the speed -- set ReframeRT to 0.5. This would result in smooth slow motion as all the frames are played back, but there are 2 times the number of frames in the source compared to rate they are being played back at. It can also be used to correct footage. If, however, you want to achieve smooth slow motion with 30fps source footage, you can use Interpolate. Interpolate I think makes up frames that don't exist, like "frame blending" in premiere. This video might be of some help, as it shows you the features in action https://www.youtube.com/watch?v=1NvrfXOa8j0.