Licensing and extension packs

[rousseauhk]

Not sure if this is the right place to post licencing questions, so apologies in advance if its not appropriate.

My understanding is that GPL code (and this would presumably include the core of VirtualBox) requires that all code that links to it (either as a static library, or by dynamic linking) also be released under GPL. However the extension packs are released under a different license. How is this the case?

And were I to say develop my own extension pack and want to sell it, would I be required to open source it under GPL?

cheers
Steve

[noteirak]

The best place for this would be the dev mailing list - pretty precise question where only the devs can actually give you a valid answer.