Brand coverage is the share of your tracked prompts where your brand appears in the AI answer at all, regardless of how prominently. If you track 100 prompts and your brand shows up in 60 of the answers, your coverage is 60 percent. It is a breadth metric: how many of the questions that matter mention you in any form.
For brands, coverage is useful because it separates two different problems. Low coverage means you are absent from many relevant questions, which is a content and presence gap. High coverage with low rank or share of voice means you appear often but weakly, which is a prominence problem. The fix differs in each case.