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Just a way to secure data access, its safe as can be, ask @fabien! :-)

But why can't I look at other people's UA without using the API? Why would I need to log in for that?

The level of UA computations is... pretty extreme. The server costs are something to consider as well. That's why we limited full UA-API access to UA Registered Partners.

Is it open source?

It isn't yet. But it is planned to go open source soon.

It is ;-)

It is, we just haven't deployed the repo(s) yet.

How much do I have to delegate to become a UA Registered Partner?

I think you just have to contact either one of the owners about your idea of using UA and they will consider adding you to that Partner program. So it is not based on delegations as far as I know. (Especially since UserAuthority is the reputation metric you want access to, and @steem-ua only is a curation service making use of UA)