Does UR816C support surround 5.1?

I´ve checked UR816C out!
I want to buy it.
I need to know, if it supports surround 5.1 or 7.1 before I buy.

That depends on the software you’re using with it.
The interface as such just presents outputs to the software, it doesn’t care if you use 6 outputs for a 5.1 setup or for 3 different pairs of stereo speakers for example.

I use Cubase.
I have read a operation manual and a startup guide but find nothing about surround 5.1
Are you sure it supports surround?

As steve77 wrote, it depends on the software. If that software can address the outputs of the UR816C separately, then you can mix in surround.
But what is your exact use case?

Yes, Cubase supports surround. I have never used it myself, but it has plugins such as surround mixers, utilities, and so forth. You need to set it up, and it will work with any interface that has multiple outputs. It can be 5.1, 7.1, 8-speaker array, you name it. Film composers use it a lot this way - see Junkie XL. So, as the other users said, it’s not about the interface. It’s about the software, and Cubase can do surround.