The Serial Wire Debug (SWD) debug interface is used for programming the firmware and accessing registers. SWD can be accessed, for example, with SEGGER J-Link debugger hardware. A standard 2 x 5 pin header interface can be found on the nRF9151 Development Kit (DK).
VREF is used to provide the reference voltage level for the SWD programmer and must be the same as the nRF9151 VDD_GPIO voltage level.
nRESET (pin 9) is a dedicated System on Chip (SoC) reset pin. An SoC reset is generated when the nRESET pin is pulled low. To ensure that the reset is issued correctly, follow the pin reset timings and instructions in nRF9151 Product Specification.
For details on access port protection, see Access port protection in nRF9151 Product Specification.