The nrf52hf_keil variant is built for SoCs with a floating point unit (FPU), while the nrf52nf_keil variant uses software floating point calculations. The hf variant is faster, but requires FPU support from the SoC. For instance the nRF52840 has an Arm Cortex-M4 with FPU, so you can use the hf variant. Some of the lower-end nRF52 devices do not have an FPU, so for them you must use the nf variant.
