Difference between Expense Report Audit Rules and Expense Report Audit Selection Rules

What is the difference between Expense Report Audit Rules and Expense Report Audit Selection Rules. I see some of the fields in both the pages are same.

if different values are added in both, which task will take precedence?

Example: Random audit percentage is set to 5 in Expense report audit rules and to 3 in Expense Report Audit Selection rules for the same business unit.