> What does it mean for the license to say "within the US"?
It means exactly what it says; you have to read the whole thing (or at least the two sentences before the CC 1.0 Universal text, which is the operative mechanism by which the second sentence is effected), not a fraction of the first sentence.
> Does this mean this software cannot be used outside the US?
No. The license explains two things:
(1) Without any license, this is automatically public domain in the US because it is a federal government work.
(2) The federal government (as the owner of the copyright at creation outside the United States, at least anywhere that applies the common rules underlying the Berne Convention) waives copyright worldwide, and does so via the CC 1.0 Universal declaration (the text of which is then included.)
So, it is, to the extent that this is legally possible, copyright-free globally.
Some countries don't recognize the concept of Public Domain works. In the US, many government works are Public Domain as a matter of law. This creates complications internationally in those countries that don't recognize the legitimacy of Public Domain as a legal concept. Nonetheless, the US still wants to make it available internationally.
To satisfy these conflicting requirements, the US government places it in the Public Domain in the US to satisfy US law. Additionally, they make it available internationally under a license that approximates the intent of Public Domain while still being recognized as a legally valid thing.
Good question. Copyright laws are country-specific, right? So perhaps it is just trying to be clear that there is no license being asserted outside of the US.
What does it mean for the license to say "within the US"?
Does this mean this software cannot be used outside the US?