Pareto’s Principle, widely known as the 80:20 rule, as applied to sales states that 80% of the sales are made by the top 20% of the reps. This rule can be extended further to state that the top 20% of this 20% makes 80% of those sales, so that the top 4% (20% of 20% = 4%) of the reps make 64% (80% of 80% = 64%) of the total sales.
Applied further yet again, the top 20% of this 4% is the elite of the elite, the top 1% (technically the math works out to 0.8%). This extremely small number of reps makes over half of the total sales (80% of 64% = 51.2%).

An interesting twist here is to apply the rule in both directions. Then it is seen that the top 20% of the bottom 80% does 80% of the sales for that group, and so on.

This chart illustrates the concept for a 100 rep team: