Usually when someone claims T3+ they mean they have UPS clusters in 3+1 (or such) configuration and two different such UPS clusters power two power-strips in a rack. Then, would also have incoming grid power supply from two different HV sub-stations with non-intersecting cable paths. They would also have diesel power generators in 3+1 or 5+2 configurations with automatic startup time in seconds. The UPS's power storage (chemical or potential energy based devices) can hold enough energy to handle full load for several minutes. If these are design and maintained correctly, even while concurrent scheduled maintenance is ongoing, an unexpected component failure should not cause catastrophic outage. At each layer (grid incomers, generator incomers, UPS power incomers) there are switches to switch over whenever there's a need (maintenance or failure).

If they claim tier4, then they basically have everything in n+n configuration.