What is a customization update?

Hey guys

I am very new to Android os and am wondering what a customization update is? I have just purchased a HTC10 and it's the first thing that came up. It says something along the lines of 'a customization update is available for your handset' and then has a little message from my carrier. It doesn't specifically say what the update does. I imagine the update will just contain some bloatware from my carrier but I want to make sure I won't be missing anything important.

The update is...

What is a customization update?

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