Pretty simple: if you fly in a balsa wood ship with light armour you have a canopy made of chocolate. If you have heavy military spec armour then canopies are much tougher.
So in practice (as an illustration)
15 (ED V3.4) is normal for default lightweight
20 is for reinforced
30 military (i.e. the pre 3.4 normal)
40 composite / mirrored
So in practice (as an illustration)
15 (ED V3.4) is normal for default lightweight
20 is for reinforced
30 military (i.e. the pre 3.4 normal)
40 composite / mirrored