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 / IVUptime: concurrent maintainability vs fault tolerance; legacy ~99.982% (~1.6 h/yr) vs ~99.995% (~26 min/yr), now Uptime-disavowed
Tier III vs IVconcurrent maintainability (any path serviceable, no load impact) vs fault tolerance (survive any single unplanned fault)