I think you are right, but if its necessary to use one of the list then you need to remove its context from dialplan in extensions_additional.conf, so that number will be free to use.
all of these numbers are viewable through the features tab of the GUI so you can see what they are and you can change their assignments if that is what you want to do.
as an added note, if you try to assign an extension to one of these reserved numbers, freePBX will prevent that with an error message telling you that number is already in use.