LFXO is a low-speed clock that can be used within EFM32/EFR32, and uses a 32.768kHz external crystal as a clock source. As a clock source, you can use not only a crystal but also an oscillator or a CMOS clock. Depending on what you use as the clock source, the frequency that can be used as LXFO will change.
 

• XTAL mode (CMU.LFXOCTRL.MODE = 0)
  

Use an external crystal. The frequency that can be used is only 32.768 kHz in order to ensure oscillation.

• BUFEXTCLK mode (CMU.LFXOCTRL.MODE = 1)

Use an external oscillator. In this case, frequencies below 32.768 kHz (eg 1 kHz) can be used. Please note that frequencies above 32.768 kHz cannot be used.

• DIGEXTCLK mode (CMU_LFXOCTRL.MODE = 2)

Uses an external CMOS clock. In this case, frequencies below 32.768 kHz (eg 1 kHz) can be used. Please note that frequencies above 32.768 kHz cannot be used.

*The above shows hardware restrictions. For example, when implementing Bluetooth on EFR32, 32.768kHz is required because the stack uses 32.768kHz for time management. Please also consider the specifications of such applications.
Please also check the Silicon Labs community site for information.