Most of the file types (files "grouped" by their extension) in Windows have an associated application which gets executed when a user double clicks a file in the Windows Explorer.
Sometimes, you will need to add (register) a custom command for a known file type. For example, you may want an option to open PAS files (Delphi units) with Notepad.
Read the full article to find out register or unregister a command for a file type (extension).
Related: Activate a Running Instance when a User Opens an Associated File (By Extension) | Get Associated application by Extension | How to Run your Application by Double-Clicking on a File (Register Extension)