All field keywords beginning with
html_form_ are passed directly into the form hash.
they are really defined in form.tt which ships with Gantry.
html_form_optional is a boolean that,
allows the user to leave the field blank.
To see an example,
bigtop -c example.bigtop all
Change to the newly created Kids directory and look in the form method in lib/Kids/GEN/Child.pm.