You first Classify a connection say Xbox (using its MAC address) is classified as "High".
Then you choose what percentage of Outbound and Inbound bandwidth the Classifications can receive and can send. Xbox bandwidth may be set to "None" (unlimited) or maybe you set it to 95% of bandwidth.
by the way DNS should be classed as "Highest" and set to "None" (as in no restriction) or 100% of bandwidth (otherwise webpage requests and other DNS requests may timeout).
So in this situation Xbox which is classed as "High" on the classification list gets priority over other classifications AND it gets 95% of bandwidth.
Think of it like the line outside a trendy nightclub where the regular ugly people are made to wait so they are classified as "Low" but here comes a celebrity who is classified as "High" (aren't they all!...anyway) and gets to go in before everyone.
That's a pretty simplified if not confusing attempt to explain it and I should say I could never really get it to work well on Tomato, torrents would always suck up all available bandwidth no matter what I tried.