as引导的非限制性定语从句,可以修饰整个主句和主句中的一部分,从句可位于主句之前,句中或主句之后,通常用逗号将其与主句隔

你说的其实是一种插入语结构,它可以放在句子的任何一个位置.有的语法专家把它归纳成定语从句.反正争议还没有停止.
例句：
1 As is known to all,China is a developing country belonging to the third world
2 China,as is known to all,is a developing country belonging to the third world.
3 China is a developing country belonging to the third world,which / as is known to all.