**type**: ``boolean``**default**: ``false``

Specify that checkboxtypeisnot a combinedtype.

This option specifiesif a form is compound. As it's not the

empty_data

**type**: ``closure``

**type**: ``string``**default**: ``mixed``

This option determines what value the field willreturn when the ``empty_value``

choice is selected.

In checkbox, the valueof ``empty_data`` is overriden by the value returned by

the datatransformer (see:doc:`/cookbook/form/data_transformers.rst`).