When it comes to iXBRL taxonomies such as ESEF, there are different types of errors; severe errors and warnings/notifications. The difference between severe errors and warnings lie in the language that describes them. Severe errors are phrased as a MUST, which means they have to be changed, while warnings are phrased with a SHALL, which is a recommendation but often not required.
Errors are blocking, which means you cannot deliver a report containing even one, but warnings/notifications are non-blocking which means you are allowed to have them in your report.