Trademarks - per the FSF
Trademarks are associated with some software. For example, the name of a program may be trademarked, or its interface may display a trademarked logo. Often, the use of these marks will be controlled in some way; in particular, developers are commonly asked to remove references to the trademark from the software when they modify it.
In extreme cases, these restrictions may effectively render the program nonfree. It is unfair for someone to ask you to remove a trademark from modified code if that trademark is scattered all throughout the original source. As long as the practical requirements are reasonable, however, free system distributions may include these programs, either with or without the trademarks.
Similarly, the distribution itself may hold particular trademarks. It is not a problem if modification requires removal of these trademarks, as long as they can readily be removed without losing functionality.
However, it is unacceptable to use trademarks to restrict verbatim copying and redistribution of the whole distribution, or any part.