When the interface MCU receives a Data Terminal Ready (DTR) signal from a terminal, it performs automatic Hardware Flow Control (HWFC) detection.
Automatic HWFC detection is done by driving P0.21 (Clear to Send (CTS)) from the interface MCU and evaluating the state of P0.19 (Request to Send (RTS)) when the first data is sent or received. If the state of P0.19 (RTS) is high, HWFC is assumed not to be used. If HWFC is not detected, both P0.21 and P0.19 can be used freely by the nRF application.
After a power-on reset of the interface MCU, all Universal Asynchronous Receiver/Transmitter (UART) lines are tri-stated when no terminal is connected to the virtual serial port. Due to the dynamic HWFC, if HWFC has been used and detected, P0.21 (CTS) is driven by the interface MCU until a power-on reset has been performed or until a new DTR signal is received and the detection is redone.
To ensure that the UART lines are not affected by the interface MCU, the solder bridges for these signals can be cut and later resoldered if needed. This might be necessary if UART without HWFC is needed while P0.19 (RTS) and P0.21 (CTS) are used for other purposes.
If you need P0.19 (RTS) and P0.21 (CTS) for another task, use SW7 to disable RTS and CTS on both UARTs.