Cut a customer base into segments that hold up in use. Use when deciding which attribute to split on, writing a segment definition someone else can reproduce, sizing a segment, setting how often it recomputes, resolving people who land in several segments at once, deciding what happens to everyone who lands in none, or retiring a segment that has stopped separating. Not RFM or behavioral scoring, not what goes inside the message, not contact collection, and not metric formulas.
