This hierarchy defines MIB entities for system level information as opposed to information valid only on a specific card type or protocol. MIBs in this hierarchy may or may not be implemented on a specific Zhone platform but are not specific to only one product. Those MIBs will be in the Zhone product MIB hierarchy.
Sources for this description
Multiple manufacturers; see the original MIB MIB copies (LibreNMS/Observium Community)