The upstop rail is usually the rail that limits the underlever's motion. The key stop rail is usually that rail that keeps the keys in place during cartage.
Therefore, some people call it the "Key Upstop Rail". I have heard the rail that Ed refers to called the "Damper [Lever] Upstop Rail".
I have also heard the subject in question called the "Shipping Rail" because it really serves little useful purpose other than to keep the keys in place when a grand piano is put on its side for shipping. It probably won't be missed until then but if it is absent, the keys may fall out of place and require a service call from a technician to get them back where they belong. I have performed any number of those service calls over the years when a technician neglected to re-install the rail.
Having said that, there have been instances where I could eliminate the "klop" sound from quickly released keys simply by adjusting the Key Upstop Rail to a close tolerance. The felt on the rail prevents the key from bouncing and therefore cuts down on some of the clatter it may make. This will sometimes work in both grands and verticals.
Care must be taken, however to make sure that the rail is not actually pressing on any keys. If you see an uneven key level, this is the first thing to check. If the Key Upstop Rail is too low, it may be pressing slightly on some keys and make them appear to be unlevel.