Authorization classes are concatenated. In other words, the higher class usually belongs to the lower class and automatically gets the rights assigned to the lower class.
The algorithm works like this:
Owner class. An object's owner may, or may not, belong to the object's group. If the owner does belong to the group, then the owner gets whatever rights are assigned to the group. The object's owner automatically belongs to the world and nobody classes, so the owner automatically gets whatever rights that object assigns to those two classes.
Group class. Members of the object's group automatically belong to the world and nobody classes, so the group members automatically get whatever rights that object assigns to world and nobody.
World class. The world class automatically gets the same rights to an object that are given to the nobody class.
Nobody class. The nobody class only gets those rights an object specifically assigns to the nobody class.
The basic principle that governs this is that access rights override the absence of access rights. In other words, a higher class can have more rights than a lower class, but not fewer rights. (The one exception to this rule is that if the owner is not a member of the group, it is possible to give rights to the group class that the owner does not have.)