UFO is a different product. You've gotten 2 different classes of polishes mixed up.
Xtra Shine is what's considered an extender polish. One can define an Extender as a polish that uses finer grit levels of their micro-abrasives than a regular polish like Reacta Shine.
UFO is called a slip agent polish. There aren't many of them. Others that I know of are Neo-Tac's Control-It and Powerhouse's (used to be Track's) Delayed Reaction. These contain "something" that clogs the pores of reactive resin coverstock, reducing their ability to absorb oil. This ability is most of what makes them hook. So balls with this polish applied will go longer but unlike regular and extender polishes, this also makes them hook LESS and have less backend. That something in Control-It and Delayed Reaction, is, I believe, silicone; once applied it is there for good OR until you sand it off with a fairly rough pad, like 500 grit. UFO is different in that it uses an organic agent (I don't know what it is exactly) to clog the pores. It last about 20 - 25 games and can be removed much more easily, like with a 4000 grit pad.
All of these will polish a ball. I have used UFO, Control-It and Delayed Reaction at times, but I tend to stay away from such compounds that affect my carry power. That's just me; that may do what you need. I would only apply them to balls that are already very weak (Tropical Storm, for instance) when conditions get really dry and you'd rather not go to a urethane.
Xtra Shine is one of my favorite polishes of all time. You can use it like any other polish and vary the results from 2 directions: the underlying grit level and the amount of polish you apply.
Hope this helps. If I can be of any further assistance, please just ask. If I haven't answered your question well enough, I'll be happy to add more, if I can.