I'm going to answer this with reservations.....if you dont know this already, you should find someone who is to help you with your project. Not trying to be a @$$, but you REALLY dont want to create electrical problems for yourself.... they can be a nightmare to resolve later...

A buss is a distribution bar for data or signals or power.. we are talking about a power buss or distribution point.....
A lug is a bolt/screw mounted on the bar with a nut holding down the wire terminals to make contact with the buss bar....

Turn on your volt meter to DC volts at the proper range to read 12V (probably 20v range)...with the ignition OFF hold the BLACK(-) probe to the ground point on the drivers fender and then touch the RED(+) probe to one of the terminals on the left side buss.... does it show 12v? if yes, that one is hot 12v all the time. If no, turn on the key and see if it has 12v. If yes, that is the switched 12v.....