For right clicking, are ctrl-click and two-finger-tapping used exchangeably? For right clicking, there are two ways when using the trackpad


Control-click, i.e. tapping the trackpad while pressing Control key 
two-finger-tapping 


Can they be used exchangeably? 

If not, when to use which?
Yes, they can be used interchangeably. The two-finger tap is a shortcut employed by OS X for more convenience / accessibility, however, it has exactly the same function as the control-click.