Live retrieval engines respond to current content updates more rapidly than training-corpus engines. A new page indexed today may be cited by Perplexity tomorrow but may not appear in ChatGPT's training-corpus retrieval until the next training checkpoint.
The IEO Engine measurement framework tracks live retrieval citations and training-corpus citations separately because they update on fundamentally different timelines. Both citation types are valuable but indicate different stages of the deployment's progression through the AI ecosystem.