Double-click in Pro Tools to create new tracks! Choose a place in the window (either mix or edit!) where no tracks exist. The different modifiers determine the type of track!
Option = Stereo Aux
Command = Stereo Audio
Shift = Stereo Master
Control = Stereo Instrument
Mix and match any/all of the above to get any two, three, or all at the same time! In other words; if you hold down Shift and Option as you double-click you will get BOTH an new Master track and an Aux.
Without hitting a modifier, double-click and you will get the last type of track again. So if you had Option held to create an Aux. Then later determine you need another Aux, just double-click, no modifier (Option) needed!