What are Custom Summary Formulas in matrix reports?

Custom Summary Formulas (CSFs) are formula-based calculated columns/rows added to Summary and Matrix reports. They operate on summary (aggregate) values rather than record-level values. Use cases: calculating gross margin percentage (Sum of Profit / Sum of Revenue), showing conversion rate (Closed Won Count / Total Opportunities Count). CSFs use functions like RowCount, SUM(), PARENTGROUPVAL() (reference a parent group's aggregate), and PREVGROUPVAL() (reference the previous group's aggregate). PARENTGROUPVAL is especially powerful — it allows percentages relative to a parent row total. CSFs only display when groupings are present (Summary or Matrix reports) and cannot be used in Tabular or Joined reports as filters.