You may have come across lots of files which you can open in different software’s. Every file extensions have certain applications they can open in. Some file extensions can open in multiple software’s too, for example a jpg file can open in the default windows photo viewer or Photoshop or a custom software you have chosen.

When you install a software sometimes it is programmed within the installation to set up the software as the default viewer for a particular or multiple extensions. For example if you install Firefox it will set itself as the default viewer for HTM and HTML files. Of course you can disallow this from happening in the advanced options while installing the software or by modifying the options in your favorite software to set it as the default one.