ISO 9001:2015 does not mention audit findings categorization. ISO 19011:2018 in clause 6.4.8 states that nonconformities can be classified according to the context of the organization and its risks. This classification can be quantitative (e.g. 1 to 5) or qualitative (e.g. minor, major). Grading nonconformities (as separate categories) is generally used only in certification audits (not so often in internal audits).
About your concerns with subjectivity, remember that this grading is based on the auditor’s judgment and experience.
Minor nonconformity – a nonconformity that does not affect the capability of the management system to achieve the intended results.
Major nonconformity – a nonconformity that affects the capability of the management system to achieve the intended results. For example, if the company completely failed to fulfill a certain requirement; if a proce ss has completely fallen apart; or if you have several minor nonconformities that are related to the same process or to the same element of your management system.