“Returning customers (%)” is an Outcome KPI because it reflects the business result of your service/product performance—customer loyalty/retention—rather than the activity performed. Inputs are resources (budget, staffing), process KPIs track how work is done (cycle time, utilization), and outputs capture what was produced (orders shipped, tickets closed). Returning customers indicates whether the outputs and experience delivered created enough value for customers to come back. It’s also commonly used at organizational or department scorecard level because it ties to growth efficiency and long-term revenue stability. Measurement challenges include defining “returning” (repeat purchase within 30/90/365 days, repeat booking, active subscription renewal) and ensuring identity resolution (same customer across channels/accounts). Documentation should specify cohort logic, time window, and the denominator used (total customers vs customers eligible to return). In KPI selection, retention outcomes should be paired with leading drivers (service quality rating, delivery performance, complaint resolution) to make improvements actionable rather than purely descriptive.