The FreeBSD-Advocacy mailing list exists to help bring together individuals who want to help promote and evangelise FreeBSD to other individuals, organisations, and manufacturers.
The FreeBSD-Chat mailing list is for general discussion that may or may not directly relate to FreeBSD. Discussions that wander off topic in the other mailing lists frequently migrate to chat, where they may drag on, and on, and on...
The FreeBSD-Doc mailing list is for discussions relating to the FreeBSD documentation. Joining the mailing list automatically makes you a part of the FreeBSD Documentation Project.
The FreeBSD-FS mailing list is for people working on implementing and improving new or existing filesystems under FreeBSD.
The FreeBSD-Hackers mailing list is for technical discussion relating to FreeBSD. Individuals working on FreeBSD can bring up problems, and discuss alternative solutions and implementations.