The basic qualification for the different track type values is the state of development and - by far less important - also maintenance. If a track is not maintained at all it will deteriorate over the time and may need a downgrading probably. In case of a track with a chipseal surface this may happen quite fast, and when the chipseal is gone the track isn’t grade 1 any longer. Anyhow the grade of development is much more important, as already described in the linked topic.
Btw: the grade of development is very closely related to the permanent surface firmness. Most tracks are created to give access to farmland or forests where soft underground is prevalent. Undeveloped soft underground remains soft, although it may be hard as rock after extensive dry periods. As soon as rains starts the firmness changes rapidly. By developing a track step by step as decribed here this dependency on weather conditions is fairly reduced. The higher the grade of development, the higher is the permanent firmness of such a track.
In so far the permanent firmness of a track reflects the grade of development.
Even if there are different understandings of how to assess tracktype, the classification works quite well, at least in my area. Most of the tracks I have checked in the past years (mapped by others) fit the scheme I described. There may be an up- or downgrade of 1 level necessary from time to time but overall the understanding of the different types appears to be same.