Rnk is a simple serial number that starts from 1 for the latest Quarter. This is used in the variables to figure out previous two Quarters. For e.g. if latest Quarter is Q4-2018, then using Rnk 2 and 3, we immediately know that previous two Quarters are Q3-2018 and Q2-2018. Similarly, when user click on any quarter from the list box we get it's Rnk value and then add +1 and +2 to get the previous two quarters. If you look at the variable definitions, it should become more clear. Thank you.