How do client and server plugins interact?

[Chriscic]

It seems cool you can install a plugin just on a client PC, but I'm confused with how it does or doesn't interact with server plugins.

I assumed any server plugin would show up on all clients, but that doesn't seem to be the case as for example Ortus trailers is installed on the server and showing up on extenders but not on my client PC.

Perhaps this has something to do with the fact that I installed the same plugin on my client PC (to try it) before removing it?

What happens if the same plugin is installed on both the server and client PC?

Already RTFM, but this is not explained. Any help appreciated. Thanks,
-Chris C.

[GKusnick]

If it's a UI mod, you must install it on the client if you want it to affect the client UI. (Placeshifter and extender UIs run on the server, so one server installation covers them all.)