When you have a video with several tracks, which track plays with the proxy?

A common question with screener playbacks is -  When you have a video with several tracks, which track plays with the proxy version of the file (versus the source file)? 

When you have 5.1 audio (6 channels) or more and the video gets converted, the proxy produced will have 2 channels. 

For example: 

 A common 5.1 channel audio configuration

When you have a video with several tracks, which track plays with the proxy?

C L R Ls Rs LFE  will be converted to 2 channel audio L and R. 

Updated March 1, 2021, based on CORE Version 6.5.1