Mix Convert plug-in, which is a newly included plug-in that can convert incoming audio in one speaker format to outgoing audio in a different format: you could use this as an insert on a stereo channel to downmix to mono, for example. Nuendo automatically uses the Mix Convert plug-in in situations where you have a 5.1 channel routed to a stereo output: double-clicking the panner automatically opens a Mix Convert plug-in editor — you don’t see it instantiated as an insert on the Mixer. Mix Convert offers quite a number of controls, such as the ability to adjust the phase of individual speaker channels (except the front centre channel, of course!), adjust the incoming gain and the level of the surrounds, centre channel and LFE that goes into the downmix.
Mix Convert is actually really useful for creating downmixes, especially since this is far more complex in Pro Tools: audio channels can only be routed to outputs with the same speaker layout — you can’t route a 5.1 channel to a stereo channel without creating sub-paths, and creating a downmix requires some creative routing if you don’t use a plug-in such as Waves’ M360° Surround Mixdown plug-in from the 360° Surround Tools bundle, which adds to the cost of the system.