这个字段，只在使用complex的权限等级时才发挥作用，它分为三个等级，各个等级说明如下：
* Level 1: requires that owner_user_id matches $this->perm_user_id
* Level 2: requires that the $owner_group_id matches the id one of
* the (sub)groups that $this->perm_user_id is a member of
* or requires that the $owner_user_id matches a perm_user_id of
* a member of one of $this->perm_user_id’s (sub)groups
* Level 3: no requirements
等级一：要求owner_user_id字段必须和perm_user_id匹配
等级二：要求owner_group_id字段必须和perm_user_id所在的group匹配，或者owner_user_id匹配一个perm_user_id，而这个perm_user_id必须是当前$this->perm_user_id所在的group的某个成员
等级三：无要求