diff -u modules/field/field.api.php modules/field/field.api.php
--- modules/field/field.api.php
+++ modules/field/field.api.php
@@ -809,7 +809,12 @@
  *   - #bundle: The name of the field bundle the field is contained in.
  *   - #field_name: The name of the field.
  *   - #language: The language the field is being edited in.
- *   - #field_parents: The '#parents' space for the field in the form.
+ *   - #field_parents: The 'parents' space for the field in the form. Most
+ *       widgets can simply overlook this property. This identifies the
+ *       location where the field values are placed within
+ *       $form_state['values'], and is used to access processing information
+ *       for the field through the field_form_get_state() and
+ *       field_form_set_state() functions.
  *   - #columns: A list of field storage columns of the field.
  *   - #title: The sanitized element label for the field instance, ready for
  *     output.
diff -u modules/field/field.attach.inc modules/field/field.attach.inc
--- modules/field/field.attach.inc
+++ modules/field/field.attach.inc
@@ -478,7 +478,9 @@
  *     $langcode => array(
  *       '#field_name' => The name of the field,
  *       '#language' => $langcode,
- *       '#field_parents' => The '#parents' space for the field in the form,
+ *       '#field_parents' => The 'parents' space for the field in the form,
+ *          equal to the #parents property of the $form parameter received by
+ *          field_attach_form(),
  *       '#required' => Whether or not the field is required,
  *       '#title' => The label of the field instance,
  *       '#description' => The description text for the field instance,
@@ -491,7 +493,9 @@
  *         '#entity_type' => The name of the entity type,
  *         '#bundle' => The name of the bundle,
  *         '#field_name' => The name of the field,
- *         '#field_parents' => The '#parents' space for the field in the form,
+ *         '#field_parents' => The 'parents' space for the field in the form,
+ *            equal to the #parents property of the $form parameter received by
+ *            field_attach_form(),
  *         '#title' => The title to be displayed by the widget,
  *         '#default_value' => The field value for delta 0,
  *         '#required' => Whether the widget should be marked required,
diff -u modules/field/field.form.inc modules/field/field.form.inc
--- modules/field/field.form.inc
+++ modules/field/field.form.inc
@@ -309,10 +309,12 @@
 }
 
 /**
- * #after_build callback for field_default_form() to store information about the form structure.
+ * #after_build callback for field elements in a form.
+ *
+ * This stores the final location of the field within the form structure so
+ * that field_default_form_errors() can assign validation errors to the right
+ * form element.
  *
- * The 'array_parents' array is used to assign validation errors to actual form
- * elements.
  * @see field_default_form_errors()
  */
 function field_form_element_after_build($element, &$form_state) {
@@ -438,15 +440,15 @@
  *   - instance: the field instance definition array,
  *   - items_count: the number of widgets to display for the field,
  *   - array_parents: the location of the field's widgets within the $form
- *     structure. This entry is populated at '#fter_build' time.
+ *     structure. This entry is populated at '#after_build' time.
  *   - errors: the array of field validation errors reported on the field. This
  *     entry is populated at field_attach_form_validate() time.
  *
  * @see field_form_set_state()
  */
 function field_form_get_state($parents, $field_name, $langcode, &$form_state) {
-  $field_parents = _field_form_state_parents($parents, $field_name, $langcode);
-  return drupal_array_get_nested_value($form_state, $field_parents);
+  $form_state_parents = _field_form_state_parents($parents, $field_name, $langcode);
+  return drupal_array_get_nested_value($form_state, $form_state_parents);
 }
 
 /**
@@ -467,12 +469,12 @@
  * @see field_form_get_state()
  */
 function field_form_set_state($parents, $field_name, $langcode, &$form_state, $field_state) {
-  $field_parents = _field_form_state_parents($parents, $field_name, $langcode);
-  drupal_array_set_nested_value($form_state, $field_parents, $field_state);
+  $form_state_parents = _field_form_state_parents($parents, $field_name, $langcode);
+  drupal_array_set_nested_value($form_state, $form_state_parents, $field_state);
 }
 
 /**
- * Internal helper function: defines the location of field information within $form_state
+ * Returns the location of processing information within $form_state.
  */
 function _field_form_state_parents($parents, $field_name, $langcode) {
   // To ensure backwards compatibility on regular entity forms for widgets that
@@ -485,14 +487,14 @@
   // @todo Remove backwards compatibility in Drupal 8, and use a unique
   // $form_state['field'][...$parents...]['#fields'][$field_name] structure.
   if (!empty($parents)) {
-    $field_parents = array_merge(array('#parents'), $parents, array('#fields'));
+    $form_state_parents = array_merge(array('#parents'), $parents, array('#fields'));
   }
   else {
-    $field_parents = array();
+    $form_state_parents = array();
   }
-  $field_parents = array_merge(array('field'), $field_parents, array($field_name, $langcode));
+  $form_state_parents = array_merge(array('field'), $form_state_parents, array($field_name, $langcode));
 
-  return $field_parents;
+  return $form_state_parents;
 }
 
 /**