Concurrent maintainability
The ability to maintain or replace any component without shutting down IT load, the defining trait of Tier III.
Current numbers
Tier III vs IVconcurrent maintainability (any path serviceable, no load impact) vs fault tolerance (survive any single unplanned fault)
III: concurrent maintainability / IV: fault tolerance + continuous coolingUptime Tier III/IV outcomes: concurrent maintainability vs fault tolerance under the standard's defined conditions