I don’t like lawyer much, however, we all have to respect law :-) Apple has released some of their code under a license called “APSL”, what’s that? To quote Robert Watson, a FreeBSD core team member, APSL was considered as something like GPL/LGPL, which is acceptable for standard-alone userspace tools, however, should be taken carefully on kernel aspect.

When some code is licensed under APSL and someone tend to import it into the kernel, then he or she will have to:

• Make the code don’t build by default. Say, not to connect that to build.
• Carefully mark the code as “APSL licensed”
  etc.

Fortunatelly, unlike some unresponsive corporation, Apple apply APSL only on software they written from scratch or heavily modified. Other licenses remain as-is so vendors of these code will easily import these improvements.