RefTeX offers two ways to create a new BibTeX database file.
The first option produces a file which contains only the entries
actually referenced in the current document. This can be useful if
the database is only meant for a single document and you want to clean
it of old and unused ballast. It can also be useful while writing a
document together with collaborators, in order to avoid sending around
the entire (possibly very large) database. To create the file, use
M-x reftex-create-bibtex-file, also available from the menu
under Ref->Global Actions->Create Bibtex File. The command will
prompt for a BibTeX file name and write the extracted entries to that
file.
The second option makes use of the selection process started by the command C-c [ (see Creating Citations). This command uses a regular expression to select entries, and lists them in a formatted selection buffer. After pressing the e key (mnemonics: Export), the command will prompt for the name of a new BibTeX file and write the selected entries to that file. You can also first mark some entries in the selection buffer with the m key and then export either the marked entries (with the e key) or the unmarked entries (with the E key).