AUCTeX can update the style information about a file each time you
save it, and it will do this if the directory TeX-auto-local
exist. TeX-auto-local is by default set to "auto/", so
simply creating an auto directory will enable automatic saving of
style information.
The advantage of doing this is that macros, labels, etc. defined in any
file in a multifile document will be known in all the files in the
document. The disadvantage is that saving will be slower. To disable,
set TeX-auto-local to nil.