What is the meaning of #XXX in code comments?

I have seen this a lot in code, even vim marks it as a special case. #TODO and #FIXME are two other fix markers vim highlights but what does #XXX mean?

XXX in a comment is usually a heads-up. It could be:

• Something that's not implemented completely correctly.
• Something that should be fixed later on.
• Highlighting a possible problem spot.
• Something you're not sure about, a question.

I've often preferred a more descriptive tag like FIXME or TODO or HACK. XXX is often used as a catch all for the above.

Searching for 'XXX' on the FreeBSD code cross reference is a good example of many of the uses. There are thousands...