this documentation should be used starting with product versions bigger then Auction Factory 1.6.9 and Ads Factory 1.5.4

class FieldType_inputBox extends FactoryFieldTypes{

var $type_name = "TextBox";
var $class_name = "inputBox";
var $has_options = 0;
var $sql_type = "varchar";
var $length = 200;
var $_attributes = null;
var $_options = null;

function display($name, $id, $css_attributes="", $css_class="", $javascript_attributes="" ){
$attributes = $this->prepareAttributesString();
$css_class = $this->prepareClassName($css_class);
return "<input type=\"text\" name=\"$name\" id=\"$id\" $attributes class=\"$css_class\"
style=\"$css_attributes\" $javascript_attributes value=\"".htmlentities($this->value)."\"  />";
}
function display_search($name, $id, $css_attributes="", $css_class="", $javascript_attributes="" ){
return $this->display($name, $id, $css_attributes, $css_class, $javascript_attributes );
}
} 

You can add your check class as the input posted above. Customize the display and show search methods adding the name of the control to the fields.config.ini in the field_list= param .

You should have a class like:
class FieldType_checkBox extends FactoryFieldTypes
and add in the field_list  "checkBox"

COLLECT MULTIPLE OPTIONS:

"overwrite the getVar method"
class FieldType_checkbox must have it's definition of the FactoryFieldTypes getVar method so the multiple options are collected all from $_POST.

"add the [] to the controller html field name "
A multiple optioned html control must have the name "name[]" instead of "name" in it's html output like bellow:
<input type="checkbox" name="mynice_name[]"  value="value1" />
<input type="checkbox" name="mynice_name[]"  value="value2" />

Note! A checkbox group is a multi option controlled by it's behavior - you can select more than one checkbox and all checkboxes refer to the same property.

PLUGIN EXAMPLE FOR A CHECKBOX FIELD:

class FieldType_checkbox extends FactoryFieldTypes{
   
   var $type_name       = "CheckBox";

   var $class_name    = "checkBox";
   
   var $has_options   = 1;
   
   var $sql_type       = "varchar";
   
   var $length       = 200;
   
   var $_attributes    = null;
   var $_options       = null;
   
   function display($name, $id, $css_attributes="", $css_class="", $javascript_attributes="" ){
   
      $this->value = explode(",", $this->value);
      $css_class = $this->prepareClassName($css_class);
      $ret = "";
      if(count($this->_options))
         foreach($this->_options as $k => $option){
            $selected = "  ";
            if( in_array($option->option_name,$this->value) )
               $selected = " checked=\"checked\" ";
            $ret.= "<input type=\"checkbox\" name=\"{$name}[]\" id=\"$id\" $selected value=\"".htmlentities($option->option_name)."\" > ".$option->option_name;
         }
      return $ret;
   }

   function getVar($name){
     
      return implode(",",FactoryLayer::getRequest( $_REQUEST, $name ));
     
   }
   
   function getValue(){
         return $this->value;
    }
   
}