Documentation for debconf VISIBLE?

The python3-debconf package (from Ubuntu bionic) installs a debconf.py module, which defines a Debconf class. The class does not explicitly define many methods, instead adding a generic handler for a list of command names, e.g. get, input, etc; these handlers issue the corresponding debconf command (GET, INPUT). One element in that list is visible. I cannot find any documentation for debconf's VISIBLE command. Playing around with DEBCONF_DEBUG I discovered it accepts two arguments; the second argument is a question ID, but I don't know what the first argument is.