Generators are considered an external redundancy to a data center. Power distribution units (PDUs), storage systems, and power feeds to racks are all internal to a data center, and as such they are considered internal redundancies.

Portability is the ability for a cloud customer to easily move their systems, services, and applications among different cloud providers. By avoiding reliance on proprietary APIs and other vendor-specific cloud features, an organization can maintain flexibility to move among the various cloud providers with greater ease. Reversibility refers to the ability for a cloud customer to quickly and easy remove all their services and data from a cloud provider. Interoperability is the ability to reuse services and components for other applications and uses. Scalability refers to the ability of a cloud environment to add or remove resources to meet current demands.

Explanation/Reference:
Explanation:
The Infinity Paradigm of the International Data Center Authority (IDCA) takes a macro-level approach to data center design. The IDCA does not use a specific, focused approach on specific components to achieve tier status. Building Industry Consulting Services International (BICSI) issues certifications for data center cabling.
The National Fire Protection Association (NFPA) publishes a broad range of fire safety and design standards for many different types of facilities. The Uptime Institute publishes the most widely known and used standard for data center topologies and tiers.