The HTML elements are put into two packages, which can be loaded separately. You choose which package you want with this menu. When you change package the HTML menu is rebuilt.
HTML 3.2 -Contains everything in HTML 3.2 as in the recommendation by World Wide Web Consortium released 14 January 1997.
Extensions - Contains everything in HTML 3.2 plus Netscape's extensions to HTML.
You have probably heard of HTML 3.0. It was a proposal for extending HTML published in March 1995. HTML 3.0 was never widely deployed. HTML 3.2, on the other hand, adds widely deployed features such as tables, applets and text flow around images, while providing backwards compatibility with the old standard HTML 2.0.