What is important when assigning an app to a .desktop launcher?

A .desktop launcher follows freedesktop.org desktop entry and menu standards. Some standard keys are:

• Name= Specific name of the application.
• Comment= A short description of the application.
• Exec= Command to launch the application.
• Icon= Icon for the application (an icon provided by your icon theme or an absolute path to an image file).
• Type= Types of desktop entries, supported ones are Application, Link or Directory.
• Terminal= true/false, whether the application runs in a terminal window.
• Categories= Categories to which the application belongs (Main Categories, Additional Categories and Reserved Categories).
• StartupWMClass= is a very useful one.

For a detailed list of specifications, visit freedesktop.org.

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P.S. Various GUI applications which can be used to create application launchers, e.g. 'Main Menu' (aka alacarte) or 'MenuLibre' (menulibre), just creates a .desktop file in an appropriate location (e.g. ~/.local/share/applications/) under the hood.