When someone asks an AI assistant about a topic, the model does not answer the literal question alone. It expands the topic into a set of related sub-questions and answers from whatever it can retrieve. Fan-out coverage measures how many of those likely sub-questions your page actually answers.
It is the difference between a page that mentions a topic and a page that covers it. A page can be about pricing and still leave the real questions unanswered: what does it cost, is there a free tier, how does billing work, can I cancel.
Topkay generates the fan-out set from your page's topic, checks each question against your content, and judges whether a passage truly answers it, not just mentions it. The questions with no answer are content gaps: real things people ask that your page does not cover.
Fix a gap by adding a short section that answers that question directly. Do it because the question is worth answering for a reader, not to inflate a score. A page that genuinely covers its topic is the one that gets cited. See how to make a page more answerable.