What does it mean to soft switch a relay?
They appear to be delaying the (mechanical) relay energization to attempt to have the relay contacts close near the zero crossing with a tolerance of +/-1ms. It's not really related to SMPS soft switching. 

The function requires a reference AC voltage to be applied so that the relay driver can be timed. They also attempt to correct for variation of switching speed of the relay with temperature using a temperature sensor, which, of course, introduces possible issues when the temperature is not stable. 

For a bit more information, see the manual section 5.5.