The term 'HOW-TO LIST HEADING' can be solved by the three letter acronym DOS, which stands for Disk Operating System. DOS is a computer operating system developed in the early 1980s by Microsoft. It was the first widely used operating system for personal computers and was the dominant operating system for PCs until the mid-1990s. DOS was a command line-based system that allowed users to interact with their computers using text commands. It was also used to create and manage files, directories, and programs.