There are not exact or common classification of non-conformities. Only ISO 19011-6.4.8: "Nonconformities can be graded depending on the context of the organization and its risks. This grading can be quantitative (e.g. 1 to 5) and qualitative (e.g. minor, major)."
ASQ Auditing Handbook (4th edition-page 99): An auditor must sort and classify facts based on the severity of the problem, the frequency of occurrence, and the risks associated with the problem. Most quality system certification/registrar auditors are trained to report nonconformances, and consequently, many auditors of registered organizations adopt the same method of reporting. A nonconformance or finding may be classified as systemic or isolated, major or minor, or it may be assigned a severity level or graded (such as A, B, or C), depending on its effect on the product, service, process, or system.
As you see classificaiton of non-conformities may be changed for each certification bodies. Each certification body has its own procedures for classification. The method you mentioned above is common pratice which is used by many certification bodies.