64 bit mixing means that n-Track will process the audio signals using a 64 bit representation ("double floating point").
64 bit processing is available in both the 32 and the 64 bit versions of n-Track.
64 bit mixing/processing can be activated using the "Settings/64 bit processing" menu command.
64 bit mixing & processing allows for higher audio quality at the cost of a slight increase in CPU usage. Some but not all of the 3rd party plugins support 64 bit processing. For an optimal use of 64 bit processing all of the plugins in the signal chain should support 64 bit processing so that the audio signals would not need to be converted back and forth between a 32 and a 64 bit representation.