In practice
Empty search results, out-of-stock product pages and “no results in your area” screens are the usual sources.
Decide per case: return a real 404 or 410 when the thing is gone, or make the page genuinely useful when it is not.
Why it matters
Search engines waste crawl budget re-requesting pages that will never have content.
Left alone, soft 404s fill the index with pages that make the whole site look thinner than it is.
Related terms