Have you received the specifications as shown in our catalog?
= Have you received the specifications as are shown in our catalog? (as是準關係代名詞)
= Have you received the specifications which are shown in our catalog?
2012-08-23 11:24:26 補充：
as, but, than可以當準關係代名詞
We will give the audience such a show as has never been seen before.
The girl wore the same red dress as her mother did.
2012-08-23 11:24:33 補充：
Thompson was as good a basketball player as has ever played the game.
Andrew will be taking a few months off, as was mentioned in the meeting.
As is often the case, Jessica comes to school late.
As is usual in the fall, it was raining that week.
2012-08-23 11:28:33 補充：
S + V + such ... as + V (or S+V)
S + V + the same ... as + V (or S + V)
S + V + as/so ... as + V (or S+V)
as is mentioned, as is often the case, as is usual等慣用語中的as