Probably exclusive as in "no-one else (that we know) has put together exactly this combination of software on a CD"
Why would they do that? The demand is higher than they can handle. If they were able to handle the demand they would just have everybody preorder, and ship when the RPIs are available. However they have switched to "register interest" to prevent their ordering system from overflowing.
Note that if someone registers interest, they have not agreed to buy it yet. So if RS opens up earlier, you're free to switch. If they needed the customers, they would allow you to preorder which makes it much harder for you to switch....
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