Every business has the one number it looks at first. Custom reports let you build it from any column of the attribution report, including spend, profit, ROAS and the platform-reported columns.
Custom metrics are formulas over those metrics with + − × ÷ and parentheses: revenue / leads for revenue per lead, orders / leads * 100 for lead-to-sale %, spend / appointments for cost per booked call. Pick a format and dividing by zero shows a dash instead of an error.