From AT&T Support Wiki
Settings > Administration > Global Resources > System Objects
The System Objects are built into the very foundations of the platform. They tend to "play by their own rules", in that they have special features that are not generally shared by other objects. At the same time, they frequently do not have features that were designed into Custom Objects.
In short, rather than being general purpose Objects, they're optimized for high performance and utility in their respective settings.
The system objects are: