Grades have no specific definition, but generally improve in quality over time. A more recent re-release of a High Grade model may be superior to the same model from several years prior, so grade alone does not entirely define the quality of a model. Kits released in the beginning of the Reiwa era (2020s) will more often than not have far better stability than a kit released towards the beginning of the Heisei era (2000s and 2010s), for example.