1. the 510+ comes with the USB Bluetooth adapter, the 510 does not.

2. the only difference between UC and MS is the *certification* for call control:
MS = Certified for MS applications (i.e. possibly built-in Teams/Skype buttons, device is also preferenced by the programmes).
UC = Unified Communication = standardised control (also works with Teams/Skype)

I was also confused, but you are probably safe with the UC variant, unless only MS apps are used.
Hope this helped & greetings