So ZCL's 4th bday is coming up soon so the past 3 days I been to writing up an article about how ZCL came to be. This will tell you how it went through it's various changes. Also included a nice sideshow of each version that was released (Except 2.5).
Enjoy and learn about it😊
http://dproj.info/the-history-of-zcl/