With respect to Bluetooth, a UUID specifies the services and characteristics that are made available by a Bluetooth device. In particular, the Bluetooth SIG has defined this UUID for its use and use with standard services:
All pre-defined services, such as a heart rate or blood glucose, use this base value and overlay a 16-bit ID in the bold, underlined digits above for that particular service. A list of available services is maintained by the Bluetooth SIG here:
If you are using one of these pre-defined services, then you can use its 16-bit ID. It's also possible to have your company purchase a 16-bit UUID for use with the Bluetooth SIG base UUID. The advantage here is your code only needs to transmit the 16-bit value rather than the full 128-bit one. This leads to power savings due to shorter transmission times.
All other uses of Bluetooth require a full 128-bit UUID. The specification indicates that the base UUID any any derivatives from it are reserved.
How does one obtain a UUID? The best would be to use a random UUID generator. There are several websites that do this. One we've found useful is: https://www.uuidgenerator.net/