If your project's salary has a default rate of 10$ per hour and you work on a task for 10h, you multiply 10$ by 10 hours to get 100$. So, your salary for working 10 hours on the task is 100$. If you apply a custom rate with a factor of 2 on the task: Total = hours (salary * factor) + (hours * extra) = 10 (10 X 2) + (10 * 0) = 10 (20) + (0) = 200$ So, you multiply the project's default salary of 100$ by a factor of 2. The task's total doubles from 100$ to 200$. If you apply a custom rate with an extra salary rate of 2$ per hour: Total = hours (salary * factor) + (hours extra) = 10 (10 X 2) + (10 * 2) = 10 (20) + (20) = 220$ Your task's total increases by 20$ to 220$.