1、as是做关系代词.指代是的：前面整个句子（多送件人可以监控的情况）
2、是的,上面说了,代整个句子做主语.
完整应该是有is的,但由于主句的谓语也是is,所以省略掉了.
不明白,请追问
若有帮助,请采纳When RTCP sender/receiver report packets arrive—describing the reception quality as seen at a particular receiver—the information they contain is stored.那这个句子中 as 后面为啥也没有isas (it was) seenit=the reception quality