So what is the difference between a League and a Competition on TheBench™.
A League defines the core rules for your game. The League defines the positions on the field, the scoring rules, the finals structure, draft/salary cap player selection and the code (eg. AFL).
A Competition belongs to a League. You can create as many Competitions in a League as you like. All Competitions will inherit the rules of your League. You will be able to tweek some minor rules in your Competition (such as squad size and the number of clubs) but for the most part the League defines the rules. A Competition has a bunch of Clubs that will challenge each other throughout the season for the premiership.
An analogy would be a regional League in the real world, the Eastern Football League for example is the equivalent to the League.
And within the Eastern Football League there exists a set of separate Competitions (or sometimes called Divisions) in which a set of Clubs fight it out for the premiership of their Competition.
Some of our users have clubs relegated and promoted between Competitions within a given League.