A. All cohorts in your project can be used as a base cohort for index reporting.
You can choose 'Everyone' or select a specific cohort to be your base cohort.
Example: If you have global audiences you might want to run an index report by GEO location, for example comparing your UK and US visitors. To do this, you would go into insights, click the index and overlap tab, run an audience report on your UK visitors cohort and filter your base cohort to select the US visitors cohort.