jw(1) calls backends like backends/html to do the "real" conversion work. The backend gets all necessary data delivered via environment variables ready to use.
This documentation describes those environment variables.
The backend is run in the directory where the output files are to be created. It should return 0 if there weren't any problem, and return a positive value otherwise.
SGML_FILE_NAME
        The bare name of the source file
SGML_FILE
        The full name and path of the source file
SGML_JADE
        The name of the parser (usually Jade or OpenJade)
SGML_ARGUMENTS
        The full argument-string for Jade or OpenJade
SGML_CATALOG_FILES
        The list of catalog files needed by Jade or OpenJade
SGML_STYLESHEET
        The style sheet to use
SGML_BASE_DIR
        The base directory of the SGML system (default is /usr/share/sgml)