reftex-enable-partial-scans

reftex-enable-partial-scans is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, re-parse only 1 file when asked to re-parse.
Re-parsing is normally requested with a `C-u' prefix to many RefTeX commands,
or with the `r' key in menus. When this option is t in a multifile document,
we will only parse the current buffer, or the file associated with the label
or section heading near point in a menu. Requesting re-parsing of an entire
multifile document then requires a `C-u C-u' prefix or the capital `R' key
in menus.

You can customize this variable.