The HTML elements are put into two packages, with 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 specification by World Wide Web Consortium released in June 1996.
Extensions - Contains everything in HTML 3.2 plus Netscape's extensions to HTML. Some of Netscape's extensions are supported by other browsers than Netscape Navigator, such as Mosaic and Internet Explorer.
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.