If you select "Tracks", you Export will only contain the selected Tracks.
Ditto for Busses - whatevever's selected will be exported
With Entire Mix, what gets Exported is dependant on your routing. If for example you have only set up one Master Bus and all your other Tracks & Busses feed through it, you'll get the entire mix exported.
If you have one or more additional busses feeding outputs 3/4 & 5/6 etc on your interface, you'll get a 6dB increase for each additional pair. This is quite a common scenario for setting up alternative headphone mixes
Your other option, and the one that I use exclusively when I want my entire song Exported is "Main Outs".
In order to make sure everything is included, I always start by doing a Select NONE (ctrl + shift + A)