More information about UART service (AT Command), Pls refer to below link
https://raytac.blog/2018/03/28/nordic-nrf52810-at-command-ble-module-by-raytac-mdbt42q-pat/
UART service, as known as NUS (Nordic UART Service) is the most common and easy way for BLE relating IoT Implement.
Customers who is familiar with Classic Bluetooth are used to call as SPP service.