Thank you for the quick reply , The stereo mix is an encoded version of the ambisonic third order mix and this is the version used for the final medium (HDM or headphones). I call this mix binaural because once it leaves Unity it is a static (headlock mix) unless you listen to it within unity with a head mounted display.
When exporting the audio to a third party application like Pro-Tools you actually need the ambisonic mix (first, second or third order) to be able to finish the work. Another use for the ambisonic mix is to be able to export your mix to the Facebook 360 Spatial Workstation to encode your final movie output for different platforms such as youtube, facebook and more...
If you want to create sound for VR360 videos you have to be able to have acces to the ambisonic mix, not the actual encoded stereo binaural mix. Please let me know if I am not clear. :)