What is COMPAT_FREEBSD in kernel config?

["Thomas Mueller" <mueller6724%bellsouth.net@localhost>]

I notice a line
options COMPAT_FREEBSD
in GENERIC kernel configuration file for i386, but not amd64 or sparc.

What does this do, and how do I run FreeBSD i386 binaries?  chroot to FreeBSD 
installation?

In case of chroot, what about device nodes?  FreeBSD has much more advanced 
way, dynamic as opposed to preassigned.

I could also ask about COMPAT_LINUX.

Tom