上記の答えのデイビッドの助けがあれば、デイビッドは気にしないでしょう。
もっと詳しく説明してみます
Mage.phpから始めて、関数ヘルパーに移動します
public static function helper($name)
{
    $registryKey = '_helper/' . $name;
    if (!self::registry($registryKey)) {
        $helperClass = self::getConfig()->getHelperClassName($name);
        self::register($registryKey, new $helperClass);
    }
    return self::registry($registryKey);
}
a) $registryKey = '_helper/' . $name;
結果: $registryKey = _helper/sales
b)check now registry key exits ot not,if not then set a registry変数
 if (!self::registry($registryKey)) {
それtrue when system is first timesはセッションになります:
c)   $helperClass = self::getConfig()->getHelperClassName($name);
つまり
  $ name = sales Mage_Core_Model_Configクラス関数getHelperClassNameに送信して、ヘルパークラスの送信パラメーターを取得します
d)次にクラスname is registry variableeを設定します
 self::register($registryKey, new $helperClass);
結果: may be proper class name Mage_Sales_Helper_Data
Mage_Core_Model_ConfigクラスのgetHelperClassName関数に移動します
a)この関数は、ヘルパークラス名文字列$ name文字列を取得します。
public function getHelperClassName($helperName)
{
    if (strpos($helperName, '/') === false) {
        $helperName .= '/data';
    }
    return $this->getGroupedClassName('helper', $helperName);
}
機能の説明:
 $helperName is sales 
ロジック: 
 if (strpos($helperName, '/') === false) {
なる true as $helperName=sales then set  $helperName=sales/data
b)  return $this->getGroupedClassName('helper', $helperName);
詳細:
call function getGroupedClassName with params 'helper','sales/data' 
and getGroupedClassName return    class name of helper class
次にgoto関数 getGroupedClassName('helper','sales/data',null)
public function getGroupedClassName($groupType, $classId, $groupRootNode=null)
{
    if (empty($groupRootNode)) {
        $groupRootNode = 'global/'.$groupType.'s';
    }
    $classArr = explode('/', trim($classId));
    $group = $classArr[0];
    $class = !empty($classArr[1]) ? $classArr[1] : null;
    if (isset($this->_classNameCache[$groupRootNode][$group][$class])) {
        return $this->_classNameCache[$groupRootNode][$group][$class];
    }
    $config = $this->_xml->global->{$groupType.'s'}->{$group};
    // First - check maybe the entity class was rewritten
    $className = null;
    if (isset($config->rewrite->$class)) {
        $className = (string)$config->rewrite->$class;
    } else {
        /**
         * Backwards compatibility for pre-MMDB extensions.
         * In MMDB release resource nodes <..._mysql4> were renamed to <..._resource>. So <deprecatedNode> is left
         * to keep name of previously used nodes, that still may be used by non-updated extensions.
         */
        if ($config->deprecatedNode) {
            $deprecatedNode = $config->deprecatedNode;
            $configOld = $this->_xml->global->{$groupType.'s'}->$deprecatedNode;
            if (isset($configOld->rewrite->$class)) {
                $className = (string) $configOld->rewrite->$class;
            }
        }
    }
    // Second - if entity is not rewritten then use class prefix to form class name
    if (empty($className)) {
        if (!empty($config)) {
            $className = $config->getClassName();
        }
        if (empty($className)) {
            $className = 'mage_'.$group.'_'.$groupType;
        }
        if (!empty($class)) {
            $className .= '_'.$class;
        }
        $className = uc_words($className);
    }
    $this->_classNameCache[$groupRootNode][$group][$class] = $className;
    return $className;
}
**この関数タスク** 
retrieve class name by class group
param   string $groupType currently supported helper
param   string $classId slash separated class identifier, ex. group/class
param   string $groupRootNode optional config path for group config
return  string
a)$classArr = explode('/', trim($classId));
結果:explode helper name(sales/data by /) for getting classs name in array $classArr
b)$class = !empty($classArr[1]) ? $classArr[1] : null;
結果:as $classArr[1] is  data then value of $class is data
c)  $config = $this->_xml->global->{$groupType.'s'}->{$group};
詳細:get config setting then $config = $this->_xml->global->helpers->sales;
d)$className = null; 
詳細:set $className  null。
e)check rewrite class exits in config.xml files, check if class is rewrite then
        you can easly get
コード:if(isset($ config-> rewrite-> $ class)){結果:if (isset($config->rewrite->data)) {
f)for default magento ,it false to goto else 
if($ config-> deprecatedNode){
 as deprecatedNode is for model($grouptype) and ,it only used for _mysql4
g) if (empty($className)) {
結果:
if sales helper not rewrite then if (empty($className)) { becomes true
h)if (!empty($config)) {
結果
:false
if (!empty($config)) {  becomes false;`
i)   if (empty($className)) {
結果:as $className is till
 null then this code is true
j)クラスの$className = 'mage_'.$group.'_'.$groupType;
結果を作成します。      
$group=sales
$groupTyp=helper
$className =  'mage_sales_helper';
k)
if (!empty($class)) {
                $className .= '_'.$class;
            }
結果:
as $class = data the this condition  is true and $className =  'mage_sales_helper_data';
l)**大文字にする   $className = uc_words($className); ここでreturn class return $ className; これはMage_Sales_Helper_Data **です
のおかげで 
デビッドマナーズとラジーブKトミー