To understand shim over bucket, shim under bucket, and tappet... Here are some diagrams to help understand if you dont know.
This would be a tappet system, using a rocker arm. These are the ones that you will get the most ticking or rattling sound. It has much more moving parts above the valve that can wear, causing a loose valve.
This would be shim over. The problem with this is at a high RPM if you get valve float, the shim can pop out and hang the valve open. Usually you will not see this type with a rev limit over 8000rpm.
this pic kinda shows how the shim can pop out... Also that you can compress the spring with special tool and adjust the shim without removing the cams.
Shim under bucket is the most reliable and allows engines to rev to a higher RPM... In this diagram you can see that the shim would be installed under the bucket and sit ontop of the valve, this is why you have to remove the cams to do an adjustment
I decided to add this too... this is a desmodromic system, what Ducati uses. This can rev much higher than a shim under bucket because it does not us a valve spring. In Moto-GP honda complained that there should be a rev limit on bikes, because the Ducati could rev over 20,000RPM with no valve springs, using an opening and closing cam and rockers instead of a valve spring to close the valve... Valve springs start to float at about 18,000 RPM and can not keep up with the high RPM.