For Info to work, the info directory must contain a file that serves as a top-level directory for the Info system. By convention, this file is called dir. (You can find the location of this file within Emacs by typing C-h i to enter Info and then typing C-x C-f to see the location of the info directory.)
The dir file is itself an Info file. It contains the top-level menu for all the Info files in the system. The menu looks like this:
* Menu:
* Info: (info). Documentation browsing system.
* Emacs: (emacs). The extensible, self-documenting
text editor.
* Texinfo: (texinfo). With one source file, make
either a printed manual using
@TeX{} or an Info file.
...
Each of these menu entries points to the ‘Top’ node of the Info file that is named in parentheses. (The menu entry does not need to specify the ‘Top’ node, since Info goes to the ‘Top’ node if no node name is mentioned. See Nodes in Other Info Files.)
Thus, the ‘Info’ entry points to the ‘Top’ node of the info file and the ‘Emacs’ entry points to the ‘Top’ node of the emacs file.
In each of the Info files, the ‘Up’ pointer of the ‘Top’ node refers
back to the dir file. For example, the line for the ‘Top’
node of the Emacs manual looks like this in Info:
File: emacs Node: Top, Up: (DIR), Next: Distrib
In this case, the dir file name is written in uppercase letters—it can be written in either upper- or lowercase. This is not true in general, it is a special case for dir.
See the util/dir-example file in the Texinfo distribution for
a large sample dir file.