The "Open With" functionality determines which application automatically opens specific file types when double-clicked. Changing this behavior system-wide means setting a new default application for a particular file format across your entire operating system, rather than making a one-time choice. This differs from simply selecting "Open With" for an individual file, which doesn't permanently change the default association.

For example, on Windows 10/11, you can set default apps through Settings > Apps > Default apps > "Choose default apps by file type." Here, you associate file extensions like ".jpg" with Photo Editor instead of Photos. Similarly, macOS users control defaults by right-clicking a file, selecting "Get Info," choosing "Open with," clicking "Change All," then confirming the new application for all similar files.

While powerful for customizing workflows, system-wide changes can be overridden by OS updates or when installing new software claiming file associations. Security implications exist, as malware might exploit this to run harmful code; always ensure the default app is trustworthy. System limitations arise when apps are sandboxed or cannot handle certain features within files correctly.