As many of the categories overlap, you often have to choose which of the categories should be the primary category of your port. There are several rules that govern this issue. Here is the list of priorities, in decreasing order of precedence.
Language specific categories always come first. For example, if your port installs Japanese X11 fonts, then your CATEGORIES line would read japanese x11-fonts.
Specific categories win over less-specific ones. For instance, an HTML editor should be listed as www editors, not the other way around. Also, you do not need to list net when the port belongs to any of irc, mail, mbone, news, security, or www.
x11 is used as a secondary category only when the primary category is a natural language. In particular, you should not put x11 in the category line for X applications.
Emacs modes should be placed in the same ports category as the application supported by the mode, not in editors. For example, an Emacs mode to edit source files of some programming language should go into lang.
If your port truly does not belong anywhere else, put it in misc.
If you are not sure about the category, please put a comment to that effect in your send-pr submission so we can discuss it before we import it. If you are a committer, send a note to the FreeBSD ports mailing list <freebsd-ports@FreeBSD.org> so we can discuss it first--too often new ports are imported to the wrong category only to be moved right away.