The shield is peace of hardware for simplified interconnection between µFR Series RS232 devices and any Raspberry Pi version. DL shield uses pins 8 (GPIO14) and 10 (GPIO15) of the GPIO header for UART communication and PIN16 (GPIO23) for reset µFR Series device. GPIO serial port (mini-UART) is disabled by default. You need to enable them for the device’s full functionality. uFCoder library version 4.2.7 and later support DL shields on Raspbian.
The detailed explanation is available in the Digital Logic shield with uFReader RS232 documentation.