it would be helpful if the exact terms of the question were more precisely defined; Ca in what context? "Ca" as an exact formatting (uppercase C, lowercase a, no spaces, or punctuation marks), or not?
the letter combination "ca" can be used to refer to a (potentially extremely large) number of things, most usually as an abbreviation.
California as an abbreviation (US postal system).
In the periodic table, this is the symbol for Calcium.
CA is the IATA airline designator for China Airlines
.ca is the Internet country code top-level domain name for Canada.
ca. is an english language abbreviation for "circa"
there is a Cả River, located in Laos and Vietnam
a large number of word combinations using the initials c. a. are possible in languages using the roman alphabet (or simillar alphabets, possessing the same letter shapes). this includes both proper names, & simple word combinations.
example: class a
a large number of entities have proper names (and/or titles) beginning with the initial combination C. A.
this includes persons, objects, places, organizations & corporate entities, among other possibilities.
(please refer to the wikipedia (english) disambiguation page "CA" for a longer, but not comprehensive, list of possible usages)