This function is only found on the Online UPS’s.
So you can feed in for example 50Hz and get out 60Hz.
Most UPS’s follow the frequency of the supply.
Most online UPS’s can be configured, either by the front panel or by software interface so that you can get a different output frequency than that of the supply.
If your UPS is set up as a frequency converter you lose the function of the static bypass facility and upon a fault occurring the UPS will shut down and drop the load.
This is because there would be a clash of frequencies.