What does "Who owns this PC?" in the Windows 10 setup mean?

Ben,

The short answer to your question is, there's no harm in picking the second option if you want to join an Active Directory (AD) domain or Azure later. Microsoft is trying to help you set up the computer correctly, that's all.

Just remember that if you're using a local account – downloads/documents/etc. in your local account don't usually transfer over to an AD/Azure account. You can still get to them as long as you are a Computer Admin on your AD/Azure account, but it's a time-consuming process.

Yes, the language is confusing, and no, it doesn't seem Microsoft has an official document for it yet. Google/Bing searched it, nothing found with an official explanation, just lots of How-To guides that don't really discuss the ramifications of each option.

If you want to add it to AD/Azure immediately or very soon, pick the first option. Otherwise, pick the second option.

Using Windows 10 Pro with a local account (second option), I have the "Join or leave a domain' and "Join or leave Azure" buttons under Settings -> Accounts -> Work Access.

More info here:

https://social.technet.microsoft.com/Forums/en-US/74217ce8-6933-4f47-8101-7ad99922125b/who-owns-this-pc-my-work-owns-it-vs-i-own-it-small-home-networrk-with-server-2012-r2-active?forum=win10itprosetup