Buyers do not ask one canonical question. They ask dozens of variations of the same intent: “best tool for X,” “X alternatives,” “is X worth it,” “X vs Y.” A prompt cluster groups those variations so you can measure visibility for the intent, not just a single phrasing.
Clusters are the unit of work in GEO. You discover the prompts your buyers actually use, group them by intent, then track mention and share-of-voice across the whole cluster. That tells you which intents you own and which a competitor owns, with enough resolution to act.