The overall design of a datacenter is often classified as belonging to “Tier I–IV”

Tier I datacenters have a single path for power and cooling distribution, without redundant
components.

Tier II adds redundant components to this design (N + 1), improving availability.

Tier III datacenters have multiple power and cooling distribution paths but only one active
path. They  also  have  redundant  components  and  are  concurrently  maintainable,  that  is,
they provide redundancy even during maintenance, usually with an N + 2 setup.

Tier IV datacenters have two active power and cooling distribution paths, redundant components  in  each  path,  and  are  supposed  to  tolerate  any  single  equipment  failure  without impacting the load.

Typical  availability  estimates  used  in  the  industry range from 99.7% availability for tier II datacenters to 99.98% and 99.995% for tiers III and IV, respectively.