If you explore the tyGraph model in edit mode, Power BI desktop, or use older reports you may come across measures with POP %. For example, Unique Page Viewers POP%, or Engaged Members POP%.
POP% - stands for Period Over Period % essentially the percentage change in your measure from the previous period to the current one.
This means that if your reports period is 7 days long it will compare the previous 7 days to your selection. If you select a 90 day report period, then it will compare the current 90 days to the 90 days prior to the start of the period.
You select a 7 day report period. You have had 100 engaged users in the last 7 days but had 80 engaged users in the previous period. This means that you experienced an % increase.
= 4.80769% increase