hkScript is HTML-Kit's default programming / scripting language, which can be used to add new functionality to HTML-Kit.
HTML-Kit Build 290 shipped with hkScript 290 (the same build/version as HTML-Kit itself). When the work on the next build of HTML-Kit started, the hkScript language was also improved to include new functions and other features. This improved version of hkScript is a built-in part of HTML-Kit Build 292, but it's not a part of the HTML-Kit Build 290 setup. To make it possible for current HTML-Kit Build 290 users to take advantage of the new features added to hkScript, a separate file was provided to upgrade the original version of hkScript that shipped with HTML-Kit Build 290. This is known as the "hkScript 290.1 upgrade."
If you're using HTML-Kit Build 292, you do not need to install the hkScript 290.1 upgrade.
If you're using HTML-Kit Build 290, you can install the hkScript 290.1 upgrade to gain access to some of the new features added to hkScript since HTML-Kit Build 290 was released. Since many plugins written in hkScript use functions and features introduced in hkScript 290.1, you may need to install it to use some of the latest plugins in HTML-Kit Build 290. You can find out if a particular plugin requires hkScript 290.1 to run by checking the "Requirements" section on the plugin page.
Again, don't forget that HTML-Kit Build 292 already includes these new features, so you need to install hkScript 290.1 only if you're using HTML-Kit Build 290.