-i is install.
-U is upgrade.
As I mentioned, if you use -U, and it doesn't have the RPM already installed, it will install it ("upgrade" from nothing). If its already installed, it will upgrade it.
In my experience, if you use -i with it already installed, it will throw an error.
You can learn more about the rpm command than you care to know by typing "man rpm" in the terminal (:
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