Interesting question.
I suspect many (maybe most) Kepler designations can be mapped to another catalogue. The Kepler project often gives the 2MASS ID (2 micron infra-red all-sky survey). And some can be mapped to other well-known catalogues - see bottom of panel to the right in:-
en.wikipedia.org/wiki/Kepler-3
Kepler has an on-board database of stars that are within its field of view (about 4 million). It has a database bigger than that - but as far as I know Kepler points to the same region of sky all the time looking for photometric changes and only about 4 million are regularly tracked.
It might make sense from a programming point of view to have constant format star identifiers (which can be large integers for Kepler). These are called Kepler_IDs (or maybe KIC for Kepler Input Catalogue). When a candidate exoplanet transit is detected this is given a KOI number (Kepler object of interest) so if we have the third planetary
candidate on the thirty second star (where a candidate was found) then this would be designated something like KOI32-03 and if confirmed as an exoplanet becomes designated of the form Kepler-18A
Meaning that it is the 18th star found to have a
confirmed exoplanet - and this is the first such exoplanet on that star (hence A). Note that not all candidates become confirmed hence numbers between the two designations can differ.
I believe that's the gist of it. Its easier to manage a new catalogue than to work with multiple disparate catalogues.
If anyone can elaborate, qualify, or correct please feel free.
Mark C.
[Ammendment - above wikipedia reference might not be the best example reference as It appears to be a KOI and does not refer to a confirmed exoplanet discovered by Kepler but I feel its still helpful]
