I'm working with a designer who has supplied some font files as swfs for sifr.
Could someone explain why this file format is being used for a font file?
Does this have to do with legal restrictions about supplying regular font files for download from a web site - i.e. by providing them as swfs they are less usable for anything except sifr?
SIFR operates by using a small flash file (the swf) to render out the text you want to replace. The font itself is actually embedded in the swf file.
You might want to read up a bit on how sifr works to understand a bit better.
In terms of legal restrictions: Many fonts are not licensed for use on the web, regardless of using sifr, @font-face or any other technique. That's just the sad fact of things currently. Here's a few that specifically are licensed for it: http://opentype.info/demo/webfontdemo.html
Others you use at your own risk.
SIFR is an alternative to embedding fonts in your page (which downloads them to the client's machine, thus violating the license agreements that come with fonts such as Helvetica, etc.).
Using SIFR, you replace the fonts you'd like with some CSS and Javascript. That code renders the proper text, via Flash, when the page loads.
Not only do you benefit from not breaking your license agreement, you don't have to worry about font formats and you get a form of graceful failure (users can still read your text, it just won't be as pretty).
Sifr allows you to replace fonts on a page without those fonts being accessible to the end user on their own machine.
The designer knew they weren't web fonts, so they gave them to you in a format you could use to build the page with the same fonts as are in the design.
http://dev.opera.com/articles/view/custom-web-fonts-with-sifr/