When your method potentially "leaks" any checked exception, that exception or an ancestor type must be explicitly announced in the THROWS clause. Checked exception = Exception or descendant.

So compiler will "check" that exposure of "checked" exceptions are announced via THROWS.

Unchecked exceptions have no requirement for THROWS. Unchecked exception = RuntimError or descendant.

Sample from few years back. Separate checked exception class for each external system and one for SQL errors.
API NVO handles each exception - or - converts it to system-specific checked exception.
API NVO handles each returned error code - or - converts it to system specific checked exception.

• Each public function in n_api_efs declares THROWS ex_efs.
  Runtime errors are transformed to ex_efs exceptions.
• Each public function in n_api_dkjord declares THROWS ex_dkjord.
  Runtime errors and error codes are transformed to ex_dkjord exceptions.
• Each public function in n_api_excel declares THROWS ex_office.
• Each public function in n_api_word declares THROWS ex_office.
  Runtime errors incl. OLE exceptions are transformed to ex_office exceptions.

int function of_processData(...)
THROWS ex_SQL   // ex_SQL + RuntimeError exposed to caller

TRY
   // Call bunch of external systems
   ...
   return 1

CATCH (ex_SQL lex_database)
   ... Database error
   THROW lex_database   // Expose to caller

CATCH (ex_Office lex_office)
   ... Word, Excel, etc.

CATCH (Exception lex_other)
   ... Electronic-Filing, DKJord, etc.

CATCH (RuntimeError lex_runtime)
   // Need to catch and rethrow explicitly
   // Otherwise, "Finally" will silence runtime errors
   THROW lex_runtime   // Expose to caller

FINALLY
   ... cleanup
END TRY
return -1