The usage of forms in templates should be documented properly. People should understand that each form has attached variables which are accessible via {{ form.vars.<var-name> }} ($form->getVar('<var-name>') respectively) or, in a form theme, via {{ <var-name> }} directly ($<var-name> respectively).

Form children, in contrast, are accessed via {{ form.<child-name> }} (alternatively {{ form.children.<child-name>, which is necessary if the child name conflicts with one of the getters in FormView) and via $form['<child-name>'] in PHP templates.

For all form types, the available template variables should be documented (for example, in a "For designers" section) thouroughly.