The Comp Newsgroup Hierarchy
One of the Big-8 groups in Usenet is the Comp.* hierarchy. The comp.* groups are ones that carry any type of topics that relate to computers. They include groups that computer professionals and hobbyists enjoy, including topics about computer science, software source (Usenet was similar to Github back in the day to share computer code), and information on hardware and software systems. Usenet hierarchies are name trees with newsgroups as their leaves. Each sub-hierarchy has a topic which is a specialization of the topic of its parent hierarchy. Some examples of this are:
Comp.compression is both a hierarchy and a newsgroup. It is a sub-hierarchy of comp.* because it contains comp. as its name prefix. It deals with data compression, which is the compression of larger files into smaller ones by compressing them. It also is a normal newsgroup where people can post articles. This last fact cannot be figured out from just the name.
Comp.compression.research is a newsgroup which is part of both the comp.* and the comp.compression hierarchy. It deals with research in the field of data compression, so it offers yet another interest in the computer hierarchy. Actually, the existence of this group makes any articles on data compression research off-topic in comp.compression, just because a more appropriate group exists.
In some parts of Usenet there are stricter distinction between hierarchies and groups. A name is either a hierarchy or a newsgroup, but never both. To reuse the above example, hierarchies comp and comp.compression would have been created, with newsgroups comp.compression.research and comp.compression.misc (misc for miscellaneous). Topic of the comp.compression.misc group would have been everything dealing with data compression that is not covered by the research group – exactly what comp.compression, the newsgroup, is used for in the above list.