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The difference between Service and Data hex formats (UUIDs) in SensorTag?

Hello, I'm trying to grok the difference in formatting between the Service and Data hex formats in the GATT chart for the sensor tag. To give a specific example, IR Temperature Service is identified as thus:

F000AA00-0451-4000-B000-000000000000

However the IR Temperature Data is identified as thus:

12:21:00:00:00:00:00:00:00:00:B0:00:40:51:04:01:AA:00:F0

Which is obviously a different format. However, in sample code the Data Characteristic seems to be accessed thusly:

F000AA01-0451-4000-B000-000000000000

Which appears to be a reversed and slightly modified version of the GATT version, above.

Can someone please explain to me

1) Why the format change?

2) What does it mean?

3) How to interpret it?

Why is one essentially a string-based UUID and the other appears to be almost an IP address hex format. And, most importantly, how do I interpret it to check the value in code. Without sample code, how would I know to get F000AA01-0451-4000-B000-000000000000 from 12:21:00:00:00:00:00:00:00:00:B0:00:40:51:04:01:AA:00:F0

Thanks!