Unless you're working with a bst file for which some helpful documentation exists that lists all available entry types, you'll have to look at the bst file and, specifically, check which FUNCTIONs define entry-type variables. E.g., if you're on a Unix or MacOS system, you could open a command window, switch to the directory that contains your bst file -- let's call it aaa.bst -- and type the following command at the command prompt:
cat aaa.bst | grep "FUNCTION {"
All lines of code that contain the string FUNCTION { will be listed.
For the file plainnat.bst, a total of roughly 90 lines will be output. The relevant results will start with the line FUNCTION {article}:
FUNCTION {article}
FUNCTION {book}
FUNCTION {booklet}
FUNCTION {inbook}
FUNCTION {incollection}
FUNCTION {inproceedings}
FUNCTION {conference} { inproceedings }
FUNCTION {manual}
FUNCTION {mastersthesis}
FUNCTION {misc}
FUNCTION {phdthesis}
FUNCTION {proceedings}
FUNCTION {techreport}
FUNCTION {unpublished}
FUNCTION {default.type} { misc }
On the other hand, the file apacite6.bst yields about 220 [!] grep search results, and the relevant ones for our purpose again start with the line FUNCTION {article}:
FUNCTION {periodical}
FUNCTION {article}
FUNCTION {magazine}
FUNCTION {newspaper}
FUNCTION {book}
FUNCTION {incollection}
FUNCTION {techreport}
FUNCTION {intechreport}
FUNCTION {lecture}
FUNCTION {thesis}
FUNCTION {phdthesis}
FUNCTION {mastersthesis}
FUNCTION {unpublished}
FUNCTION {misc}
FUNCTION {literal}
FUNCTION {manual}
FUNCTION {booklet}
FUNCTION {inbook}
FUNCTION {inproceedings}
FUNCTION {conference}
FUNCTION {proceedings}
FUNCTION {default.type} { misc }
Incidentally, all bst files I've ever encountered contain the function default.type as well as something that makes default.type a conduit to the @misc entry type.
output.bibitembut that is just by convention not a reserved function name – David Carlisle Oct 18 '22 at 18:56@miscas the fallback entry type. – Mico Oct 18 '22 at 21:07FUNCTION { default.type } { misc }– David Carlisle Oct 18 '22 at 21:59output.bibitemcould be a valid solution, for a lack of a better one not relying on conventions. @Mico answer also rely on conventions, but there are more helper functions after the FUNCTION {article} so IMHO is not as reliable. – JackOA Oct 19 '22 at 12:11