I just bought a "96"caprice classic. but on the title,under model it has cls/impss. which is it a caprice classic or a impala ss

Pop open your trunk. On the inside of the lid, you should see a 'SPID' label (basically a sticker with a bunch of 3 digit codes).
If you see a code that says
WX3 - Impala SS package
9C1 - SEO vehicle, Police car (Caprice)
B4U - Performance package, sport (Caprice)

If you don't see any of those codes, it is a regular civilian Caprice.