The Zope Toolkit started as “Zope 3”. Zope 3 was the intended successor to the Zope 2 platform. It didn’t work out that way. Zope 3 can be used, but Zope 2 is still alive and well, and in fact started to use some parts of the Zope 3 codebase. We also have the Grok project came along that reused Zope 3 to build a related but separate web framework.
We realized that the term “Zope 3” was overloaded, meaning both the set of libraries shared by Zope 2 and Grok, and the actual web application server with a user interface that you can install. We therefore introduced the term Zope Toolkit so we could think about this set of shared libraries independently and manage them that way.