Once you start developing plugins in Plug'n Script you most likely desire to make a custom GUI for your plugin. This is when you need to learn KUIML.
KUIML is a language based on XML, so it has much in common with HTML used for web-development. KUIML was developed by Blue Cat Audio for their plugins. It has evolved a lot since the first version released in 2006, and while writing this tutorial in March 2021 there's version 2.7.1 already. Looking at Version history it's easy to see how and why new features arrived, synchronized with releases of new Blue Cat plugins as well as implementing some user requests. One can find a lot of KUIML tricks and examples looking at how other Blue Cat Audio plugins/skins are made.