A loop in the old skool sense is two numbers, a low and a high, looped together. When you call the low you hear a milliwatt tone and when you call the high you hear silence. When both numbers are called you can talk to eachother. Loops never used to supervise so it was a free call. The reason that the SoCal is called a "loop" is because there is a high and a low. If no one is on and you call the low, yo'll hear a milliwatt. Usually, loops only allowed two people on, but the SoCal is multiuser, technically making it a bridge. My definitions are a loop has a high and low, one side being a milliwatt, a bridge is one number with multiuser capabilities, and a conference is where you have commands, such as mute, unmute, finding out how many people are on, etc. Although, a more classic defenition is that if there are multi-user capabilities, it is a bridge. A conference is technically when a bunch of people are talking on a bridge. This is too confusing...they are all bridges.