In bootloader mode the tag advertises itself with name "RuuviBoot" and it provides BLE DFU Service. Nordic Semiconductor provides libraries for iOS and Android to interface with the service.
SWD (Serial Wire Debug)
If you're developing your own firmware or need to flash hunderds or more of RuuviTags, you should consider using a wired programmer for higher programming speed. To create your own hex image for flashing, you need 4 parts:
The softdevice is given by Nordic Semiconductor or other company which provides the radio protocol.