Google's documentation for each rich result type lists required and recommended properties. Missing a required property makes the item ineligible for that feature, and Google's tools report it as an error. Missing a recommended property is a warning: the item can still qualify, but the result may show less information. These are Google's rules, layered on top of Schema.org, which itself has no required properties.
How the rules differ by type
Requirements vary widely. Article has no required properties at all. Breadcrumbs require position and name for each item. Software apps require name, a price in offers and a rating or review. Events require name, startDate and location. Always read the specific page rather than relying on a general checklist.
Treating warnings sensibly
Recommended properties are worth adding when you have real, visible data for them. Do not fill them with placeholders or invented values to clear warnings. An empty warning is better than an inaccurate value, which can breach Google's guidelines.
Requirements change
Google updates requirements, adds properties and retires features. When a type is deprecated, its requirements no longer matter for Google. Recheck documentation for your key types periodically, and watch Search Console for new warnings, which often follow documentation changes.
Example
Clearing errors, leaving honest warnings
Wolfbarrow's event pages show errors for missing location and warnings for missing performer and image. The team fixes location, since every webinar has a registration URL, and adds images. It leaves performer empty for sessions without named speakers rather than inventing one.
Common mistakes
Placeholder values
Filling fields with generic text to satisfy a tool misrepresents the page.
Assuming Schema.org rules equal Google rules
Valid Schema.org markup can still miss Google's requirements for a feature.
Questions
No. Warnings only mean optional details are missing from structured data.
