It's usually a series where everyone buys in to the series and then everyone is given a numbered tag that you must display on your bag. Then people are allowed to challenge other members with the goal of getting the lowest tag number; bag tag "1" is the best. If you win your challenge you exchange tags, if someone else challenges you and beats you again you exchange tags. Usually leagues have a rule like you can only challenge people who have a tag within five numbers of yours. Also there is usually a rule about how many challenges and when/where your challenges are allowed to take place, with the people running the series deciding that.
Sometimes my mind boggles. It's so deep my mind actually boggles.