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The first invitations sent out to users of the Spotify music service include unlimited music, Spotify representatives confirmed Thursday. But in a few weeks, the company will pare that down.

Spotify didn't exactly clarify what the service offered in its press release announcing the service on Thursday. "With an invite, enjoy on-demand, buffer-free access to over 15 million songs on your computer, great social features, manage your own music files through Spotify, and sync with your cellphone or iPod," the company said. "Features occasional advertising."

But, for now, the service does offer unlimited music streaming, a Spotify representative said. (Read our Spotify review, here).

Why is this important? Because in the United Kingdom, France, Spain, and the other European companies Spotify serves, the service comes with a number of caveats attached to "Spotify Open," the free Spotify service there. In other words, U.S. users are currently being offered a better deal than European users, at least for a limited time.

If you have been able to download the free version of Spotify - either via an invitation or the social media ranking service Klout (whose perk has been temporarily put on hold) - the version of Spotify you'll see does not have any counter or time limits attached to it.

"After a few weeks, Spotify Free will be the same as the free service in Europe," the Spotify representative said.

But exactly when this will take place, or if the free service will have exactly the same limitations as European users, isn't quite clear. "There wil be at some point limits to that [free] service," the Spotify representative said, when asked to clarify.

What limits could there be? Spotify's Spotify Open FAQ on its European site provides a guide.

"When you open your account you receive 20 hours free streaming time - that's roughly equivalent to to 25 albums of music," according to the Spotify Open FAQ. "For the first six months, you will then receive 5 additional hours free streaming time every week. The extra 5 hours is added on roughly the same day of the week, and time of day that you created your Spotify account. If you do not use all of this streaming time each week, it will be saved up, to a maximum of 20 hours."

"After 6 months, your free streaming has to be limited further," Spotify added. "The weekly streaming allocation becomes 2.5 hours, and the maximum saved time will be 10 hours. In addition, each individual track can only be streamed 5 times in total. We hope that this will still give you a great opportunity to share and discover new music, and to keep your musical options wide open."

Here's where it gets a bit confusing: Spotify's initial invitations to its Spotify Free U.S. service include unlimited streaming, the key capability of its $4.99/mo Unlimited version. Spotify Unlimited eliminates both time limits on streaming music the in-app advertising. Spotify is also offering reviewers Premium subscriptions to test out, which normally cost $9.99/mo - meaning that they're not going to see the free version that most users will try out. The Premium tier offers offline modes and the ability to stream music to a mobile phone.

As for the frequency of the ads, I can't say with certainty. I've streamed a few songs at random, manually selecting each one. I've only experienced one ad, for Coca-Cola, which popped up and took up a good chunk of my monitor window.

So here's the bottom line, if you have a free invitation to Spotify, enjoy it; what will be offered to you and other users in a few weeks will be much more limited.

For more from Mark, follow him on Twitter @MarkHachman.