Support for Indie Authors discussion

Kindle Unlimited is a subscription service that Amazon offers to readers. As a self-published author, the only way to have your book available in KU is to be enrolled in KDP Select, which gives Amazon exclusive distribution rights to your ebooks. You are then paid a small amount for each borrow, just like the lending program through Prime.

I am assuming then that Amazon decides if they want to put you in KU. Or do you have to click a button when you upload your book to Amazon and choose KDP Select?

No, you decide. You chose whether you want exclusivity and KU is one of the benefits of Select.