In more detail
Where AI Overviews add a summary above the classic results, AI Mode replaces the results page with a back-and-forth conversation. A searcher can ask a long, complicated question, get a written answer with links to sources, and then ask follow-ups without starting again.
Google's documentation on AI features explains that AI Mode may use query fan-out: it breaks a question into several related searches, runs them, and builds one response from what it finds. That means a page can be shown for a sub-topic of the question even if it would never rank for the full question typed into normal search.
Google states that the same requirements apply as for AI Overviews. To be eligible as a supporting link, a page must be indexed and eligible to appear in Google Search with a snippet. There's no special markup, file or tag for AI Mode, and the usual preview controls (nosnippet, max-snippet, noindex) also limit how content appears in it.
Clicks and impressions from AI Mode are counted in Search Console's Performance report within the normal "Web" search type. They are not broken out separately.
Why it matters when hiring an agency
AI Mode can reduce clicks for some informational searches, so a good agency will talk about it when setting expectations, especially for content-heavy sites. Ask how they would adapt your content for searches that are likely to be answered in AI Mode, and how they report on it given that Google doesn't separate the data. Treat claims of a special "AI Mode optimisation" technique with caution: Google's own guidance is that standard SEO fundamentals are what count.