|What is a "location" ?||
It's a position that could be displayed on an object (generally speaking, a planet). Showing capitals on the Earth, the land on Mars of Pathfinder, the land on Moon of Apollo 11, etc.
They are joined together in files ssc, themselves categorized according to planet. It is just needed to put these files in the repertory "Extra" of Celestia, but in my research of the pure rationalization; -) I've created a directory named "Locations" inside the directory "Extra".
To display the locations in Celestia, it's pretty easy: look for the key "&" (shift+6, according to the keyboard). You can access a lot of complementary options in "Locations" menu, like change the filter, choose a minimum size to display (related to demographic density), and so on.