AUCTeX manual has provides:
5.2 Multifile Documents
AUCTeX keeps track of macros, environments, labels, and style files
that are used in a given document. For this to work with multifile
documents, AUCTeX has to have a place to put the information about the
files in the document. This is done by having an
placed in the directory where your document is located. Each time you
save a file, AUCTeX will write information about the file into the
auto directory. When you load a file, AUCTeX will read the
information in the
auto directory about the file you loaded and the
master file specified by
TeX-master. Since the master file (perhaps
indirectly) includes all other files in the document, AUCTeX will get
information from all files in the document. This means that you will
get from each file, for example, completion for all labels defined
anywhere in the document.
AUCTeX will create the
auto directory automatically if
non-nil. Without it, the files in the document will not know
anything about each other, except for the name of the master file.
Next section gives more information:
5.3 Automatic Parsing of TeX Files
AUCTeX depends heavily on being able to extract information from the
buffers by parsing them. Since parsing the buffer can be somewhat
slow, the parsing is initially disabled. You are encouraged to enable
them by adding the following lines to your
(setq TeX-parse-self t) ; Enable parse on load.
(setq TeX-auto-save t) ; Enable parse on save.
The latter command will make AUCTeX store the parsed information in an
auto subdirectory in the directory each time the TeX files are
stored. If AUCTeX finds the pre-parsed
information when loading a file, it will not need to reparse the
buffer. The information in the
auto directory is also useful for
multifile documents, since it allows each file to
access the parsed information from all the other files in the
document. This is done by first reading the information from the
master file, and then recursively the information from each file
stored in the master file.
The variables can also be done on a per file basis, by changing the
file local variables.
%%% Local Variables:
%%% TeX-parse-self: t
%%% TeX-auto-save: t
Even when you have disabled the automatic parsing, you can force the
generation of style information by pressing C-c C-n. This is often the
best choice, as you will be able to decide when it is necessary to
reparse the file.