Definition
Query fan-out (or sub-query decomposition) is the technique modern engines like Perplexity and Profound's reverse-engineering surfaced where ONE user prompt becomes MULTIPLE retrieval calls. A buyer asking 'best CRM for small agencies' triggers fans like 'what is CRM', 'CRM features for agencies', 'CRM pricing', 'CRM alternatives'. Each retrieves separately, and the citations are aggregated.
User: 'best CRM'. Engine fans to: ['what is a CRM', 'how do CRMs work', 'CRM alternatives', 'CRM pricing']. Each gets ~5 citations. Final answer pulls from the union.
How to optimize
Your page must answer the SEED query AND its likely sub-queries. VectorCite's query expander auto-decomposes seeds into 4 intent variants (informational, transactional, comparative) so the site audit catches gaps.