What is the difference between field validation and widget validation

Excerpt of README.txt

--------Configuration ---------------------------------------

FIELD: CCK Validation
* Validation process -- Select this option to process validation on the node field's
  widget values or/and the node field's field values. When both are selected validation
  is run twice.
  <This setting can be overwritten by widget's $form["force_validation_process"]!>
  
  Example:
  ========
  Field value of date selection field
  	[field_date] => Array ( [0] => Array ( [value] => 2005-01-01T02:04:00 )
  Widget value of date selection field
		[field_date] => Array ( [0] => Array ( [value] => Array ( [mday] => 1 [mon] => 1 [year] => 2005 [hours] => 2 [minutes] => 4 ) )

short: The difference is in the given format of the field's data

In CCK fields the data is passed through a various of levels. CCK differs by two states: field and widget (see cck's hook_field_settings() and hook_widget_settings() )

read: database => cck:field => cck:widget => form
write: form => cck:widget => cck:field => database

• The 'field' settings manage how to save the data to and read it from the database, because the database fields have a fix structure. The field also provides a common prepared format for the 'widget', so the widget can display the content in its way.
• The widget is the element, that says how to style the field input (e.g. checkbox, textfield, dropdown) => form. So the widget may need a different format of data. E.g. Instead of the date's timestamp or date's dateformat, it needs values for Day, Month and Year, so it can give it to the dropdowns 'day', 'month' and 'year'. This is given by an Array ( [mday] => 1 [mon] => 1 [year] => 2005)
• So there is a difference between the variable passed to the field's 'field' and to the field's 'widget'
• For both states CCK offers the opportunity to validate, so there might be differences in the given format, as it is in a date field
  I think it also appears in multiple value text fields, where field is array('value'=>array(0=>...,1=>...)) and widgetarray(0=>array('value'=>...),1=>array('value'=>...))