quartile returns the quartile of the selected value relative to all other values of the field in the report.
quartile([Customer], [Metric]")
would calculate the quartile for Metric for each unique value of Customer in the report.
sum returns the sum of Metric for all rows in the report.
sum([[Metric])
would calculate the sum for Metric across all rows in the report.
sum([[Metric], [Region])
would calculate the sum for Metric per unique Region in the data.