In a book I have, there is a following classification of defect:

1. Critical : A defect receives a “critical” severity level if one or more critical system functionalities are impaired by a defect with is impaired and there is no workaround.
2. High: A defect receives a “high” severity level if some fundamental system functionalities are impaired but a workaround exists.
3. Medium: A defect receives a “medium” severity level if no critical functionality is impaired and a workaround exists for the defect.
4. Low: A defect receives a “low” severity level if the problem involves a cosmetic feature of the system.

To be honest, I do not get it.. For example point 2. What if fundamental but not critical feature is impaired and there is NOT a workaround. The same for point 3: what if no critical functionality is affected but there is no workaround? E.g. optional field in the registration form does not work. No workaround but barely an issue.