The Territorial cities are 32 cites which can be built by any nation after that nation has built its first 16 cities. Historically, these are either cities like Mecca and Tarsus which have changed hands so often that it is hard to attribute them to one nation or another, or they are cities like Toronto and Sydney which are a part of nations which were simply not large or powerful enough to include in Civilization as whole civilizations.
Once a civilization has built all of its cities as well as every city on this list, it can build no more cities. Attempts to build a 49th city will produce an error message. Note that 48 is the limit, regardless of whether you actually choose to use the names presented on these lists.