Can Someone explain the difference between Zscripts and Zplugins?

I assume files with the extension .zsc are plugins?

Can these be edited?

There are a number of archived plugs that would be super useful if they would work.

What has me baffled is some older ones work other do not?

What was changed between versions to break them?

Mostly interested in the Alpha/Texture changer based on the Gnomon one.. But there are others also...

Is there a easy way to port them to new versions of Zbrush?

I'll keep on reading the zscripting info in the mean time...