This is the so-called hybrid mode. Enclosed is the definition in English:

There is also a switch for Hybrid mode - allowing the fan to be disabled completely when the unit drops to a specific temperature threshold. Seasonic claim the Hybrid Fanless mode works up to:

Approximately 50 % (±5 %) of system load at 25 °C.
Approximately 30 % (±5 %) of system load at 40 °C.


Short German summary: When hybrid mode is activated, the fan remains off if the power supply unit is operated either at 50% load at less than 25°C or at 30% load at less than 40°C.